MEMS deformable mirrors (DM) have many merits of low drive voltage, high response speed, small power consumption, low cost and small size. Its surface shape and displacement versus applied voltage are significant factors of MEMS DM. Phase-shifting interferometer (PSI) has many advantages such as non-contact, quickness and high precision. A phase-only liquid crystal spatial light modulator (LC-SLM), as a linear phase-shifter in PSI, is linear calibrated for its phase-shift characteristics. The PSI is set up to measure the static characteristic of MEMS DM. Five-step phase-shifting method is used to calculate the phase distribution from interference fringes, and Global phase unwrapping algorithm to solve the holes, noise and breakpoint of interfere images. Compared to the measurement results using Zygo instrument, these two experimental results are very close. The experiment results show, this measuring system is very reliable, convenient and cheap. Moreover, this test system need not stitch some fringe images to get the whole surface shape of the mirror like the Zygo instrument.