A lateral shearing interfeometer for the wavefront test of small beam is proposed. The interferometer is composed of a polarization beam splitter, a crystal plate, a quarter-wave plate, a mirror and a CCD detector. The polarization beam splitter is used both as a polarizer and an analyzer. The interferometer can test wavefront of low coherent light for its equal optical path length. Small shearing amount can be obtained by the crystal plate whose optical axis is parallel to its surface, thus it′s suitable for the wavefront test of small beam. The shearing amount can be continuously adjusted by rotating the crystal plate. Using ASAP software, the lateral shearing interferometer is simulated. The simulation result is well coincident with the theoretical derivation. In the experiments, the interferometer is built to obtain interferogram by rotating the crystal plate. The experimental result is well coincident with the simulation result. The usefulness of the interferometer is verified.