Nowadays, as a new type of optical fiber devices, with flexible structure characteristic and steady frequency-selecting performance among the key support device areas of optic fiber technology, Fiber Bragg Grating (FBG) is rapidly developed as an very important basic device.Therefore, the study of phase mask plate which will be good for the mass production of the optical fiber gratings is becoming a central issue. The appearance of phase mask plate technology makes the industrialized production of fiber optical gratings with a stable wavelength possible. This paper based on the optical fiber gratings written online is mainly about the study of related servo control system. By the design of the servo control system of phase mask plate, we will get the ideal purposes that phase mask plate is precisely positioned, torque of plate is increased and the time of picking any plate is undifferentiated.