求助，关于post同步执行的问题

代码如下，除了省份选择了，其它的都选不中。我觉得是post的数据还没获取到，就已经执行了下一步的代码。有什么办法吗？
<script>
$.ajaxSetup({async:false});
$(function(){
    if (<?php echo $tbuser['province']?>) {
        $("#province").attr("value",'<?php echo $tbuser['province']?>');
        selectcity();
        $("#city").attr("value",'<?php echo $tbuser['city']?>');
        //selectcounty();
        //$("#county").attr("value",'<?php echo $tbuser['county']?>');
    }
});
function selectcity() {
    $.post("<?php echo url('mkpoint/getcity');?>",{parentid:$("#province").val()},function(data){
        $("#county").html("<option value=''>选择区县</option>");
        $("#city").html(data);
    });
}
function selectcounty() {
    $.post("<?php echo url('mkpoint/getcounty');?>",{parentid:$("#city").val()},function(data){
        $("#county").html(data);
    });
}
</script>

                   <select name="province" id="province" onchange="selectcity();">
                        <option value="">选择省份</option>
                        <?php
                        foreach($provinces as $province){
                            echo "<option value='".$province['area_id']."'>".$province['area_name']."</option>";
                        }
                        ?>
                    </select>
                    <select name="city" id="city" onchange="selectcounty();">
                        <option value="">选择城市</option>
                    </select>
                    <select name="county" id="county">
                        <option value="">选择区县</option>
                   </select>