A dual-beam heterodyne interferometer is designed in this paper. Unlike the Zeeman-effect laser, the two frequencies are generated by two acousto-optic modulators. The different frequency beams are spatial separated, but the optical path comply the principle of common path. Nonlinearity in this system is analyzed in this paper. The periodic nonlinearity error caused by non-ideal laser source is not existed in this system. The stability of this system is tested.