放一个定时器
inerval设置为1000
enabled设置为true

定时器tick代码
textBox1.Text = (int.Parse(textBox1.Text) - 1).ToString();

下面这个方法也可以用
for(int i=20;i>=0;i--){
textBox1.Text = i.ToString();
Thread.Sleep(1000);
}

private void timer1_Tick(object sender, EventArgs e)
{

        label1.Text = (int.Parse(label1.Text) - 1).ToString();
        if ((int.Parse(label1.Text) <= 0))
        {
            timer1.Stop();
            button1.Text = "开始";
            button2.Enabled = true;
            MessageBox.Show("倒计时完毕！");
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        if (timer1.Enabled)
        {
            timer1.Stop();
            button1.Text = "开始";
            button2.Enabled = true;
        }
        else
        {
            timer1.Start();
            button1.Text = "停止";
            button2.Enabled = false;
        }
    }

    private void button2_Click(object sender, EventArgs e)
    {
        textBox1.Text="0";
    }