The operating characteristic of Mach-Zehnder modulator is influenced severely by environmental factors such as temperature changing. Therefore, a bias controller is required to stabilize the operating point automatically. Applying a dither signal and detecting its harmonic wave to measure the position of operating point is a useful method. However, when the amplitude of modulation signal is large, the prior art method fails to lock the bias point. The harmonic wave resulting from modulation transfer function has been simulated and analyzed. It is shown that previous harmonic wave calculation formulate will not be suitable when a large modulation signal inputs. Therefore, we have modified the calculation method and proposed a bias controller with new architecture to maintain the working point in large modulation signal condition. At last, we have developed corresponding circuit, and test result shows that our method can lock the working point within accuracy of 1°, which is quite useful in large modulation signal application.