To solve the problems of manufacturing large aspherical primary mirror, an innovative idea was put forward: According to the characteristics that ultra-thin mirror can be easily deformed, a spherical mirror which is easier to fabricated and test, is employed to replace the aspherical mirror, then it is deformed into an aspherical one by adaptive optics technology. The predigested model, i.e., bended beam used in qualitative analysis was proposed, its deflection formula under the freely supported condition was set up. Based on an off-axis aspherical primary mirror, the relationship of
aspherical surface error (RMS) in tangential plane with bended beam's radius R, position S of concentrated load P and geometry length L of arc was analyzed. The range of R and S satisfied surface accuracy were calculated. Otherwise, aspherical surface error in sagittal plane was found, and bended beam's radius R, position S of concentrated load P and length L of arc which were suited for aspherical figure accuracy of both tangential plane and sagittal plane were given.