<select name="pWaqqqy" id="sel_type" onchange="selectFunction()">
        <option value="">全部</option>
        <option value="7">aa</option>
        <option value="8">bb</option>
    </select>
    <select name="pWay" id="sel_children0" style="display:none;">
        <option value="">全部</option>
        <option value="5">a</option>
        <option value="3">a</option>
        <option value="1">a</option>
        <option value="2">a</option>
        <option value="6">a</option>
    </select>
    <select name="pWay" id="sel_children1" style="display:none;">
        <option value="">全部</option>
        <option value="4">b</option>
    </select>

jQuery.fn.removeSelected = function() {
    this.val("");
};

function selectFunction(){
            if($("#sel_type").val()=="7"){
                 $("#sel_children0").show();
                 $("#sel_children1").hide();
                 $("#sel_children1").removeSelected();
             }else if($("#sel_type").val()=="8"){
                 $("#sel_children1").show();
                 $("#sel_children0").hide();
                 $("#sel_children0").removeSelected();
             }else{
                 $("#sel_children0").hide().removeSelected();
                 $("#sel_children1").hide().removeSelected();
             }
        }