The phase-shifting Jamin shearing interferometer is very fit for measurement of diffraction-limited wavefront. To improve the performance of existing phase-shifting Jamin shearing interferometers with two polarizers, a phase-shifting Jamin shearing interferometer based on polarization combination is proposed. In the interferometer, the beam to be measured is reflected by two surfaces of the Jamin plate to form two interfering beams. They are polarized and combined to form a polarization phase-shifting interferogram. The interferometer keeps up the optical configuration and improves fringe visibility. Two kinds of phase-shifting Jamin doube-shearing interferometers before and after improvement are built for experiments. Results show that the usefulness of the interferometer is verified.