protected void Page_Load(object sender, EventArgs e)
{
     BindDropDownList1();
     BindDropDownList2(DropDownList1.SelectedValue);
}

private void BindDropDownList1()
    {
        IList<Class1> list = BL.Instance.GetDropDownList1DataSource();
        
        DropDownList1.Items.Add(new ListItem("---请选择---", "-1"));
        foreach (Class1 class in list)
        {
            DropDownList1.Items.Add(new ListItem(class1.text, class.value));
        }
    }

    private void BindDropDownList2(int value)
    {
        DropDownList2.Items.Clear();
        DropDownList2.Items.Add(new ListItem("---请选择---", "-1"));
        if (value > 0)        
        {               
            IList<Class2> list = BL.Instance.GetDropDownList2DataSource(value);
            foreach (Class2 class in list)
            {
                DropDownList2.Items.Add(new ListItem(class.text, class.value));
            }
        }
    }

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {        
        BindDropDownList2(BindDropDownList1.SelectedValue);
    }

------解决方案--------------------
再在Page_Load里面要加一个if (!IsPostBack) 哈.

------解决方案--------------------
参考:
http://blog.csdn.net/insus/archive/2007/11/11/1878690.aspx

http://download.csdn.net/source/282546