GridView1.Rows[0].Cells[0].FindControl("textbox1")；

------解决方案--------------------

我用的是datagrid。。
我接触web也没几天。。感觉和Gridview差不多，说错了勿喷。。
C# code

<asp:DataGrid runat="server" ID="Dg_BusiStore" AllowCustomPaging="True" AutoGenerateColumns="False"
        AllowSorting="True" CellPadding="4" DataKeyField="Id" BackColor="White"
        BorderColor="#AED0EA" BorderStyle="None" BorderWidth="1px" Width="100%" 
        CssClass="ui-widget gridview" onitemcommand="Dg_BusiStore_ItemCommand">
<Columns>
<asp:TemplateColumn HeaderText="盘点数量" HeaderStyle-Width="100px">
                <ItemTemplate>
                    <asp:TextBox ID="Txt_CheckStorage" runat="server" Style="text-align: right" MaxLength="8"
                        Text='<%#Eval("CheckQuantity") %>' Width="100px"></asp:TextBox>
                </ItemTemplate>
            </asp:TemplateColumn>
            <asp:TemplateColumn HeaderText="盘点金额" HeaderStyle-Width="100px">
                <ItemTemplate>
                    <asp:TextBox ID="Txt_CheckAmount" runat="server" Style="text-align: right" ReadOnly="true"
                        Enabled="false" AutoPostBack="false" Width="100px" Text='<%#Eval("CheckAmt") %>'></asp:TextBox>
                </ItemTemplate>
            </asp:TemplateColumn>
            <asp:TemplateColumn>
                <ItemTemplate>
                    <asp:Button ID="Button1" runat="server" CausesValidation="false" CommandName="Save" Text="保存"
                        CssClass="fg-button ui-state-default ui-corner-all" />
                </ItemTemplate>
            </asp:TemplateColumn>
</Columns>
</asp:DataGrid>

protected void Dg_BusiStore_ItemCommand(object source, DataGridCommandEventArgs e)
        {
            if (e.CommandName == "Save")
            {
                string Id = Dg_BusiStore.DataKeys[e.Item.ItemIndex].ToString();
                DataGridItem Dg_Item = Dg_BusiStore.Items[e.Item.ItemIndex];
                TextBox Txt_Storage = Dg_Item.FindControl("Txt_CheckStorage") as TextBox;

                if (string.IsNullOrEmpty(Id))
                {
                    this.Master.ShowErrorDialog("没有找到盘点信息,保存盘点数量失败");
                    return;
                }
                int intStorage;
                if (!int.TryParse(Txt_Storage.Text.Trim(), out intStorage))
                {
                    this.Master.ShowErrorDialog("你输入的“盘点数量”有误,请输入正整数");
                    return;
                }
                if (!UpdateCheckStorage(Id, intStorage))
                {
                    this.Master.ShowNotice("保存盘点数量失败");