In the field of precision measurement, He-Ne lasers are the prior choice to manufacture laser interferometer, which takes the wavelength as the scale in measurement. Therefore, the frequency stability is of great significance. The theories of frequency stabilization in dual-frequency lasers was studied and the isocandela points of the parallel and vertical light were utilized as frequency stabilization points. By designing thermal servo control circuit based on light intensity balance and controlling in both analog and digital methods, the frequency stabilization in Zeeman-birefringence He-Ne dual-frequency lasers was accomplished. Beat frequency experiments were carried on a Zeeman-birefringence He-Ne laser with 7.95 MHz frequency difference, the frequency stability index can reach the level of 10-9. The uncertainty in frequency was better than 1.074×10-8(k=2) in repeated measurements. Meanwhile, the stablity of the frequency difference was measured. The variation of the frequency difference was within 8 kHz and the relative deviation was 0.001, which can totally meet the demands of commercial dual-frequency laser interferometers.