In The paper a new method which can mark the placement of the fast or slow axis ofwave plate is sugges-ted. The measured wave plate is placed between polarizer and analyzer, and measures the intensity of the emergent light for two group data through revolving it, with that can mark one of the op tics principal axises of the measured wave plate. Then,measuring the op tics principal axis is the fast or slow axis through judging the state of polarization of the emergent light. The method have not any require to the wavelenghth of the lamp-house, and to the linearity, un- dercurrent, isotropy of the photoelectric detector . It can mark the placement of the fast or slow axis of the wave plate having any phase delay, the precision ofmarking the placement of the fast or slow axis is to be 0. 1 degree.