DropDownListFor如果要创建多选列表，则不要使用。你使用ListBoxFor助手。

查看型号：

public class MyViewModel
{
    public string[] SelectedIds { get; set; }
    public IEnumerable<SelectListItem> Items { get; set; }
}

控制器：

public ActionResult Index()
{
    var model = new MyViewModel
    {
        // preselect the first and the third item given their ids
        SelectedIds = new[] { "1", "3" }, 

        // fetch the items from some data source
        Items = Enumerable.Range(1, 5).Select(x => new SelectListItem
        {
            Value = x.ToString(),
            Text = "item " + x
        })
    };
    return View(model);
}

看法：

@model MyViewModel
@Html.ListBoxFor(x => x.SelectedIds, Model.Items)