The force-exerting laser with tunable frequency difference has a broad application prospect, there are few reports on the thermal drift of frequency difference of this laser, but it is an important application index, especially for the interferometer of lithography machine. The frequency difference of birefringence Zeeman dual frequency laser was assigned through elastic force method, and its states of frequency difference in the drift stage, transition stage and stable stage were observed. The experiments prove that the stability of frequency difference is better than 23 kHz/h, and its repeatability is better than 130 kHz; In addition, this paper also comparatively analyzed the influences of elastic force-exerting elements with different structures or materials on frequency difference. The experiments indicate that the uniformity and stability of the temperature distribution play an important role in the frequency difference thermal drift.