The modulation of RF signal matters a lot to the performance of laser transmittion.For meeting the requirements on the indexes such as noise factor,the RF signal above the C band mainly applies M-Z modulators to modulate RF signal.In this paper,the methods of bias control in M-Z modulators were discussed and the pilot frequency was mainly studied.Starting from the problems of the method applied in radar and communication systems,the existing control methods were improved and new bias control circuitry was designed.Tests results and practical application indicate that the method of pilot frequency can meet the requirements of practical applictions.