将你的纯javascript函数修改为这个函数（这不影响已经选择的选项）DEMO jsfiddle

     window.onload = function AddItem(text, value) {

        // Get all Drop Down/List Box in document
        var sel = document.getElementsByTagName("select");

        for (var x = 0; x < sel.length; x++) {

           // Create an Option object and set it's value/text
           var opt = document.createElement("option");
           opt.text = 'Please select...';
           opt.value = 'Please Select';

           //prepend in select box
           sel[x].insertBefore(opt, sel[x].options[0])
        }
     }

$(document).ready(function() {    
   $('<option>', {
      text     : 'Please select...',
      value    : 'Please select...',
      selected :  true
   }).prependTo('select');
});

http://jsfiddle.net/Gsp9b/

或者：

var select = document.getElementsByTagName("select"),
    opt = '';
for (var i = 0; i < select.length; i++) {
    opt = document.createElement("option");
    // opt.selected = true;
    opt.text = 'Please select...';
    opt.value = 'Please Select';
    select[i].options.add(opt);
}

干得好：

$('select').prepend('<option>Please select...</option>');

$(document).ready(function() { 
    $("select").prepend("<option value='' selected='selected'>Please select</option>");
});