比如第1个选为省分，第2 个为相对的市，

如图：

代码：

Imports System.Data
Imports System.Data.SqlClient
Public Class WebForm1
    Inherits System.Web.UI.Page

    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        If Not IsPostBack Then
            Dim con As SqlConnection
            Dim cmd As SqlCommand
            Dim dr As SqlDataReader
            con = New SqlConnection("server=(local);uid=sa;pwd=;database=lab")
            cmd = New SqlCommand("select * from test1", con)
            con.Open()
            dr = cmd.ExecuteReader
            DropDownList1.DataSource = dr
            DropDownList1.DataTextField = "proName"
            DropDownList1.DataValueField = "proID"
            DropDownList1.DataBind()
            dr.Close()

            con.Close()

        End If

    End Sub

    Private Sub DropDownList1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged
        Dim cmd2 As SqlCommand
        Dim dr2 As SqlDataReader
        Dim con2 As SqlConnection
        con2 = New SqlConnection("server=(local);uid=sa;pwd=;database=lab")
        cmd2 = New SqlCommand("select * from test2 where proID=" + DropDownList1.SelectedValue, con2)
        con2.Open()
        dr2 = cmd2.ExecuteReader
        DropDownList2.DataSource = dr2
        DropDownList2.DataTextField = "cityname"
        DropDownList2.DataValueField = "cityID"
        DropDownList2.DataBind()
        dr2.Close()
        con2.Close()
    End Sub
End Class