In order to reduce the sensitivity to external disturbance of four-mode differential laser gyro (FMDLG), an adaptive path length control system is designed to make the FMDLG run at optimal operating point. Path length of the FMDLG is modulated with small amplitude sine wave. The amplitudes of 1st and 2nd harmonic signals in light intensity are extracted with phase sense detector technique. The above two signals along with light intensity are used to realize adaptive path length control, which consist of automatic gain control, automatic modulation control and path length control. Experimental results show that adaptive path length control reduces the influence of variations of optical and electronic components parameters as well as light intensity on control system. It is important that it can make the FMDLG run at optimal operating point steadily. The adaptive path length control helps to reduce FMDLG′s sensitivity to magnetic field.