Abstract. Fatigue failure was a typical failure form of mechanical structure parts, elastic modulus drop was an important characteristic of structural material occur fatigue, so a new mechanical structure fatigue monitoring online method was proposed. Firstly, a test device that could impose cyclic load with adjustable frequency and amplitude on cantilever beams was designed; secondly, a stress measurement system based on Fiber Bragg Grating sensor to monitor structural state of fatigue was developed; thirdly, a method to calculations stress based on this system was described; finally, the online method was used in the elastic modulus drop.