The invention provides a method for locking a LiNbO3 Mach-RPAMZM off set working point. When a modem is not provided with radio-frequency signals, proper detecting voltage is added at the offset input end. A photoelectricity detector is used for detecting light intensity at the output end of the modem. The light intensity is input to the microprocessor as feedback signals. The microprocessor can calculate voltage corresponding to any phase position on a transfer function of the modem at present. In order to reduce influence of laser power fluctuation on the feedback signals, largest output light intensity of the modem is calibrated with a certain period. According to the method, no extra disturbance signals need adding, no extra noise can be caused, and any phase position point can be locked. Through selection of the proper testing voltage, the feedback signals are enabled to be influenced to a minimum degree by system noise. Locking precision can be greatly improved. The method is simple in device, convenient to operate, and high in stability. Moreover, the laser power fluctuation and light path consumption influence the system little.