textbox1.ID = "textbox1";
        textbox1.AutoPostBack = true;
        test2.Controls.Add(textbox1);

        test1.InnerText = textbox1.Text.ToString();

    <label id="test" runat="server"></label>
    <label id="test1" runat="server"></label>

protected void Page_Load(object sender, EventArgs e)
        {

                createTextBox();
        }


        private void createTextBox()
        {
            TextBox txt = new TextBox();
            txt.ID = "Txt_value";
            txt.AutoPostBack = true;
            txt.TextChanged += new EventHandler(txtchange);
            ph_text.Controls.Add(txt);
        }

        private void txtchange(object sender,EventArgs e)
        {
            foreach(Control con in ph_text.Controls)
            {
                
                if (con.GetType().Name.Equals("TextBox") && con.ID == "Txt_value")
                {
                    TextBox txt=(TextBox)con.FindControl("Txt_value");
                    lbl_value.Text = txt.Text;
                }
            }
        }

------解决方案--------------------
C# code
protected void Page_Load(object sender, EventArgs e)
    {
        TextBox tb = new TextBox();
        tb.ID = "textbox1";
        tb.AutoPostBack = true;
        form1.Controls.Add(tb);
        tb.TextChanged += new EventHandler(tb_TextChanged);
    }
    protected void tb_TextChanged(object sender, EventArgs e)
    {
        Label1.Text = (form1.FindControl("textbox1") as TextBox).Text.ToString();
    }