A common-path laser interferometer is introduced. Its most important feature is that two optical arms are entirely in a common path. Laser Doppler and laser polarization interference technologies are used in the proposed optical system. An experiment setup is established. The primary experimental result shows that this common path method is very feasible. The resolution is 0.72 nm within a 0.01 mm range, and the highest velocity is 0.56 mm/s. There is much room to improve its performance.