Based on the theory of geometric optics, the synthesis aberration equation of multibeam scanning and imaging system is derived. The primary factors, such as the angle between the incident ray and the axis of imaging system, the number of the facets of polygon, that cause bow of the off axis scanning line are analyzed, and the weight factor of each single aberration is discussed. The specific scanning lens is analyzed to satisfy the straightness of the off axis scanning line on the basis of synthesis aberration equation of multibeam scanning and imaging system.