private void button1_Click(object sender, EventArgs e)
           {
panel1.Clear(); // 清空的是当前panel上的子窗体  
               ChildForm child = new ChildForm();
               child.TopLevel = false;
              child.Dock = System.Windows.Forms.DockStyle.Fill;
              child.FormBorderStyle = FormBorderStyle.None;
              //child.Parent = this.panel1;
              this.panel1.Controls.Add(child);
              child.Show();
           }

------解决方案--------------------

非MDI
C# code

            if (panel1.Tag != null && panel1.Tag is Form)
            {
                ((Form)panel1.Tag).Close();
            }
            Form2 frm = new Form2();
            frm.TopLevel = false;
            frm.Parent = panel1;
            panel1.Tag = frm;
            frm.Show();