public partial class MainForm : Form
{
   public Form formA=new Form();Form formB=new Form();
}
public partial class FormA:Form{
   public void Load(){
     MainForm1.formB.Show();
}
}

------解决方案--------------------

原来是3个

主窗体跳转A窗体： private void button1_Click(object sender, EventArgs e)
 {
 FormA frmA = new FormA(this);
 frmA.Show();
 this.Hide();
 }

A窗体跳转到B窗体：
 private void button1_Click(object sender, EventArgs e)
 {
 FormB frmB = new FormB();
 frmB.Owner = this.Owner
 frmB.Show();
 this.Hide();
 }
B窗体跳转到主窗体：
 private void FormB_MouseDoubleClick(object sender, MouseEventArgs e)
 {
 if(this.Owner != null)  
    this.Owner.Show();
 else
{
 FrmMain frmMain = new FrmMain();
 frmMain.Show();
 this.Hide();
}
 }