A common-path interferometry is proposed to measure the phase of liquid crystal spatial light modulator (LCSLM), which is based on the pricinple of liquid-crystal polarization splitted light. The linearly polarized input beam is divided into two orthogonal polarized beams by LCSLM, and then propagates to the Wollaston prism (WP) along the same direction. Two beams transmitted from the WP generate lateral shearing and a tilt modulation is introduced into the two beams, and hence a carrier-frequency interferogram is obtained after passing analyzer polarizer. The relative phase-shift between two frames of fringes is calculated accurately by using the Fourier-transform algorithm. A phase modulation experiment to the transmissive LCSLM is given. The proposed method is less sensitive to vibrations and air turbulence. Furthurmore, it can measure the whole phase modulation characteristics of LCSLM, and the measuring result is more precise and scientific.