string sql="Select * from 系别表";   
  DataTable Deptdt = DBHelper.GetTable(sql);
            combobox1.DataSource = Deptdt;
            combobox1.DisplayMember = "系别号";
            combobox1.ValueMember = "系别名称";


   private void cboDept_SelectedIndexChanged(object sender, EventArgs e)
        {
            string sql="select * from 班级表 a,系别表 b where a.系别号=b.系别号 and a.系别号 =combobox1.selectedValue";   
  DataTable ClassDt= DBHelper.GetTable(sql);
            combobox2.DataSource = ClassDt;
            combobox2.DisplayMember = "班级号";
            combobox2.ValueMember = "班级名";
        }

------解决方案--------------------
5楼的DBHelper是另为定义的类，你把他看作得到了这张系表表