A new method for the determination of fast axis of wave-plate is introduced. Based on the principle of electro-optic phase compensation in the wave-plate phase measuring method, the relation between the polarization extinction system out-put signal and the modulating signal can be acquired through the composition of frequency fixed alternating voltage modulation signal. Through the analysis of this process with Jones matrix, the out-put signal was usually found to be the composition of fundamental frequency and second harmonic frequency, the composition signal reflects the axis information as well as the phase information of the wave-plate, and this phenomenon has never been noticed before. Through the analysis of this phenomenon, the transformation trends of the composed wave-form by rotating of wave-plates with different orientations were deduced, and through the observation of the transformation trend, one can easily distinguish the fast and slow axes of wave-plates with phase retardation >π as well as those with phase retardation <π, and their phenomena are reverse to each other.