The problem whether plane system imaging induces astigmation is researched. For the operation surface in plane system is flat, incidence beam is parallel, outgoing beam is parallel, so the astigmation can not be induced. In general, astigmation is not induced during plane system imaging process. According to research, for plane reflecting mirror, homocentric beam reflected by plane mirror is still homocentric and astigmation is not in. duced. But for prism system and parallel plate, ideal imaging can be performed only at the paraxial condition. When off-axis angle is larger, astigmation will be induced and homocentric beam passing prism system and par. allel plate is not homocentric any more.