According to requirements of the system and special imaging characters of ellipsoidal surface,a design methode-using off-ellipsoidal surface to achieve beam sampling and perfect imaging is presented.With design input,the initial configuration parameter is calculated,and the conic function of off-ellipsoidal surface is obtained.Calculation results are substituted to optical design software ZEMAX to verify imaging quality.The result shows that the off-ellipsoidal surface splits the objective point and the imaging point,reduces the length of the system,gets a perfect imaging on the imaging point,and transfers the off-axis imaging to a axis imaging,which can achieve the high quality imaging of the back focal spot system easily.