A multi-beam interferometer displacement measuring system has a light source module, a resonator module and a detecting device. The light source module has an emitter and a polaroid sheet. The emitter emits a non-polarizing beam. The polaroid sheet receives and transforms the non-polarizing beam into a polarizing beam. The resonator module receives the polarizing beam and has a coated glass panel, a corner cube prism and a wave-delay plate. The coated glass panel receives and reflects the polarizing beam. The corner cube prism receives and reflects the polarizing beam to the coated glass panel to form a resonant cavity. The wave-delay plate is mounted between the coated glass panel and the corner cube prism to receive the polarizing beam. The detecting device faces the coated glass panel to receive the interferential stripes formed in the resonator module and has a polarizing beam splitter, two power detectors and a signal processor.