加載省市：

window.onload = function () {
            var datas = {
                "吉林": ["長春", "四平", "松原"],
                "山東": ["青島", "濟南", "煙臺"],
                "山西": ["大同", "太原", "運城"]
            };

            var def = '吉林';
            //加載省份
            var s1 = document.getElementById('se1');
            for (var key in datas) {
                //創建option標籤
                var opt1 = document.createElement('option');
                opt1.innerHTML = key;
                opt1.value = key;
                opt1.selected = key == def ? true : false;
                s1.appendChild(opt1);
            }

            //加載城市
            var s2 = document.getElementById('se2');
            for (var i = 0; i < datas[def].length; i++) {

                var opt2 = document.createElement('option');
                opt2.innerHTML = datas[def][i];
                opt2.value = datas[def][i];
                s2.appendChild(opt2);
            }

            //根據省份加載對應的城市
            s1.onchange = function () {

                //獲取當前選中的省份
                //alert(this.value);

                //清空城市  再加載城市
                while (s2.firstChild) {
                    s2.removeChild(s2.firstChild);
                }
                for (var i = 0; i < datas[this.value].length; i++) {

                    var opt3 = document.createElement('option');
                    opt3.innerHTML = datas[this.value][i];
                    opt3.value = datas[this.value][i];
                    s2.appendChild(opt3);
                }
            };
        };
      <select id="se1">
      
    </select>
    <select id="se2">
     
</select>