A control algorithm to achieve working bias stability for Mach-Zehnder Modulator (MZM) is proposed. Its utilize first-order and cotangent value of Average Optical Power (AOP) with FPGA technique. Firstly, we describe the reason for stabilizing the MZM at the common point. Then, control theories for keeping the bias point at different working point are studied by numerical simulation. At last, arbitrary working point control system is built. Experiment results indicate that, the fluctuation of output average optical power is less than ±5% (within one hour) under the condition of 5 dBm laser output power (1 550 nm), 4.2 V half-wave voltage, 3 dB insertion loss and 1Gbps/1Mbps speed. The system has the ability to maintain the bias at any point in the optical power curve and improved the reliability of MZM in optical communication.