The optical axis is one of the most important parameters in the application of wave plates. In the transmission mode of spectroscopic ellipsometer, taking the advantage of Jones matrix to analyse the phase difference of P and S directions in the process of spinning wave plate, a new method for the determination of optical axis of quartz wave plate was designed. The method has characteristics of simple light path structure and high efficiency in the judging of the optical axis, and this method thus got a good practicability.