In order to minimize the bias sensitivity of a four-mode differential laser gyro (FMDLG) to external interference, an optimal operating point real-time control technique is adopted. Dispersion equalization and path-length control offset are used. The magnetic field of gain zone and detune frequency are sinusoidally modulated, then the error in the bias of FMDLG is demodulated. By negative feedback it is operated at the only optimal point on which the sensitivity to interference is smallest. The optimal operating point means the minimum bias sensitivity to magnetic field and path-length control error. In comparison with traditional path-length control method, the optimal operating point real-time control technique shows a great reduction of the gyro magnetic sensitivity and bias instability, which proves its effectiveness，and static and dynamic test results prove its correctness.