By detecting and counting the circuit, the photoelectric signal of laser gyro is amplified, shaped and the phase of it is discriminated. The pulse that are signals proportional to the rotating angle of gyro are obtained. Since the output signals of gyro are weak photoelectric ones, and shot noise and thermal noise disturbances exist in the detecting circuit, it is crucial to improve signal-noise ratio to ensure normal function of the counting circuit. To improve the signal-noise ratio of the detecting circuit, we have to guarantee that the output optical power is as large as possible. The optimal transmission of mirror for optimal optical power is derived and calculated according to the power formula of laser gyro, and experiments are followed to verify it. Both theoretical calculation and experimental results prove that, by using optimal transmittance, the output optical power of the mirror can be effectively improved.