var linkDatas = {
      provinces:[
        {
          "code":"0",
          "name":"请选择"
        },
        {
          "code":"1",
          "name":"北京"
        },
        {
          "code":"2",
          "name":"天津"
        },
        {
          "code":"3",
          "name":"河北"
        },
        {
          "code":"4",
          "name":"湖北"
        },
        {
          "code":"5",
          "name":"广东"
        },
        {
          "code":"6",
          "name":"其他"
        }
      ],
      citys:{
        0:[
          "请选择"
        ],
        1:[
          "朝阳区",
          "海淀区",
          "东城区",
          "西城区",
          "房山区",
          "其他"
        ],
        2:[
          "天津"
        ],
        3:[
          "沧州",
          "石家庄",
          "秦皇岛",
          "其他"
        ],
        4:[
          "武汉市",
          "宜昌市",
          "襄樊市",
          "其他"
        ],
        5:[
          "广州市",
          "深圳市",
          "汕头市",
          "佛山市",
          "珠海市",
          "其他"
        ],
        6:[
          "其他"
        ]
      }
      };

function addOptions(target,options){
        var optionEle = null,
            target = target,
            option = options,
            optionLen = options.length;
        for(var i = 0;i < optionLen;i++){
          optionEle = document.createElement('option');
          optionEle.value = option[i].value;
          optionEle.text = option[i].text;
          target.options.add(optionEle);
        }
      }

pro.onchange = function(){
          console.log(this);
          var ct = city[this.value],
              ctLen = ct.length,
              ctBox = [];

          c.innerHTML = "";  
          /*添加城市*/  
          for(var j = 0;j < ctLen;j++){
            ctBox.push({
              "text" : ct[j],
              "value": ct[j]
            });
          }
          addOptions(c,ctBox);
        }

<div class="content">
      <h3>下拉框联动效果</h3>
      <p>省份：
        <select name="provinces" id="provinces">
          
        </select>
      </p>
      <p>
        市：
        <select name="citys" id="citys">
          
        </select>
      </p>
    </div>

var linkDatas = {
      provinces:[
        {
          "code":"0",
          "name":"请选择"
        },
        {
          "code":"1",
          "name":"北京"
        },
        {
          "code":"2",
          "name":"天津"
        },
        {
          "code":"3",
          "name":"河北"
        },
        {
          "code":"4",
          "name":"湖北"
        },
        {
          "code":"5",
          "name":"广东"
        },
        {
          "code":"6",
          "name":"其他"
        }
      ],
      citys:{
        0:[
          "请选择"
        ],
        1:[
          "朝阳区",
          "海淀区",
          "东城区",
          "西城区",
          "房山区",
          "其他"
        ],
        2:[
          "天津"
        ],
        3:[
          "沧州",
          "石家庄",
          "秦皇岛",
          "其他"
        ],
        4:[
          "武汉市",
          "宜昌市",
          "襄樊市",
          "其他"
        ],
        5:[
          "广州市",
          "深圳市",
          "汕头市",
          "佛山市",
          "珠海市",
          "其他"
        ],
        6:[
          "其他"
        ]
      }
      };
      function addOptions(target,options){
        var optionEle = null,
            target = target,
            option = options,
            optionLen = options.length;
        for(var i = 0;i < optionLen;i++){
          optionEle = document.createElement('option');
          optionEle.value = option[i].value;
          optionEle.text = option[i].text;
          target.options.add(optionEle);
        }
      }
      function provincesCitysLink(pro,c){
        var LD         = linkDatas,
            provinces = LD.provinces,
            city       = LD.citys,
            initCity  = city[0],
            proBox    = [];

        /*添加省份*/
        for(var i = 0;i < provinces.length;i++){
          proBox.push({
            "text" : provinces[i].name,
            "value": provinces[i].code
          })
        }    
        addOptions(pro,proBox);

        /*初始化城市*/
        addOptions(c,[{
          "text" : initCity,
          "value": initCity
        }]);

        /*添加联动事件*/
        pro.onchange = function(){
          console.log(this);
          var ct = city[this.value],
              ctLen = ct.length,
              ctBox = [];

          c.innerHTML = "";  
          /*添加城市*/  
          for(var j = 0;j < ctLen;j++){
            ctBox.push({
              "text" : ct[j],
              "value": ct[j]
            });
          }
          addOptions(c,ctBox);
        }
      }
      var provinces = document.getElementById('provinces'),
          citys     = document.getElementById('citys');
      provincesCitysLink(provinces,citys);