This paper proposes an automatic bias control algorithm for dual parallel Mach-Zehnder modulator which is proposed for 16 Quadrature Amplitude Modulation（16QAM）formats to solve the working points drifting problem in dual parallel MachZehnder modulators.The algorithm is divided into two parts

which are initialization part and tracking locking part.The purpose of the initialization part is to find a working point close to the bias voltage to reduce the time required to lock the working point in the tracking locking algorithm.The tracking and locking parts use the detection method of dithering signal to find accurate working bias point

and track the offset point varying in real time.Therefore

the bias voltage can always stay at the best bias point with small the range of variation.The factors that affect the results of the algorithm are analyzed in the experiment verification.The measured data show that this algorithm can find the best working bias point of dual parallel Mach-Zehnder modulator in a relatively fast time.It also shows the ability to track and adjust the working bias point in real time.