$("#id").multiSelect({ oneOrMoreSelected: '*',selectAll:false,noneSelected:'默认显示' });

Usage: $('#id').multiSelect( options, callback )

 Options:  selectAll          - whether or not to display the Select All option; true/false, default = true
           selectAllText      - text to display for selecting/unselecting all options simultaneously
           noneSelected       - text to display when there are no selected items in the list
           oneOrMoreSelected  - text to display when there are one or more selected items in the list
                                (note: you can use % as a placeholder for the number of items selected).
                                Use * to show a comma separated list of all selected; default = '% selected'
           optGroupSelectable - whether or not optgroups are selectable if you use them; true/false, default = false
           listHeight         - the max height of the droptdown options

// Update the textbox with the total number of selected items, and determine select all
$("#id").updateSelected() 

// Update the dropdown options
$("#id").multiSelectOptionsUpdate()

// Hide the dropdown
$("#id").multiSelectOptionsHide()

// Show the dropdown
$("#id").multiSelectOptionsShow()

// get a coma-delimited list of selected values
$("#id").selectedValuesString

使用javascript 设置多选框的默认选中项。

$("#id").attr("value","");//清空选中项。

var ids='1,3,6';//设置选中框ID。
           var id_Ojbect=(ids).split(",");//分割为Ojbect数组。
           var count=$("#id option").length;//获取下拉框的长度。
           for(var c=0;c<id_Ojbect.length;c++){
             for(var c_i=0;c_i<count;c_i++) 
                {
              if($("#id").get(0).options[c_i].text == id_Ojbect[c]) 
                   { 
                       $("#id").get(0).options[c_i].selected = true;//设置为选中。
                   } 
               }
           }