public partial class Form1 : Form
{
private int num;

public Form1()
{
InitializeComponent();
this.BackColor = Color.White;
}

public void Display(Graphics g,int num)
{
SolidBrush b = new SolidBrush(Color.Red);
g.FillRectangle(b, 20, 20, 5+num, 5);
}

public void UnDisplay(Graphics g,int num)
{

SolidBrush b = new SolidBrush(Color.White);
g.FillRectangle(b, 20, 20, 5+num, 5);
}

private void Form1_Load(object sender, EventArgs e)
{

}

private void timer1_Tick(object sender, EventArgs e)
{
Graphics g = this.CreateGraphics();
this.Show();
num = 0;
Display(g, num);
UnDisplay(g, num);
num++;
}
}

汗死，你的num运行以后永远都等于0。等么可能移动啊…………
你应该把你的初始赋值NUM改成全局变量，在那里初始化你的变量才可以。

代码改动如下：

public partial class Form1 : Form
{
private int num;
num=0;
........

private void timer1_Tick(object sender, EventArgs e)
{
Graphics g = this.CreateGraphics();
this.Show();
//num = 0; （这里可以直接删除掉）
Display(g, num);
UnDisplay(g, num);
num++;
}
}