Lock-in is important to rate-biased laser gyro, but accurate lock-in measurement is always difficult. According to lock-in equation of laser gyro, approximate relation of lock-in and laser gyro output signal harmonic is derived from theoretical approximation, and based on this approximate relation the harmonic measurement for laser gyro lock-in is provided. Modification and errors analysis are processed through accurate numerical result of lock-in equation. Finally lock-in value of a laser gyro is measured by harmonic method, and the result shows that the measurement accuracy can be better than 5%.