The quantitative analysis about a laser scanning imaging system for rotating polygon is theoretically described. The polygon scanner is located in front of the scan lens in order to guarantce the scanning image point in a flat field. It is important in system designing to determine the positional relationships between the incident beam at mirror, the rotation axis of the polygonal and the optical axis of the optical system when the angle of incident laser beam is 2α relative to the optical axis of the system. the calculation formulas of the scanning angle, scanning efficiency and the entrance pupil drift of the system are derived and the relationships between scanning angle, entrance pupil drift of system and many important factors (such as the width of the incident beam, incident angle and the geometrical size of the polygon, etc.) are discussed in details. The results show, in order to increase the scanning efficiency of the system, the incident angle of laser beam should be small. And the range of scanning efficiency from 0.4 to 0.7 is more reasonable in the system designing.