Mach-Zehnder electro-optic modulators (EOMs) with parallel arms are easy to be affected by the temperature change and other environmental factors,which results in unstable performance of the modulator.In this manuscript,we propose a novel method for automatic bias voltage controlling of the modulator. A small sine voltage signal is first input to the modulator as a disturbing signal.Then,by monitoring the slope chan ge of the output signal,the drift direction and quantity of the working point can be quickly determined.The desig ned circuit is used for demonstration in the pulse modulations.Our experiments show that the fluctuatio n of extinction ratio (ER) of the modulated signal can be controlled within 0.26dB.This proposed method can al so be applied to lock modulators working at any point of their transfer curves and cancel out the instability of the input light power.