The performance degradation of fiber Bragg grating strain sensors is unavoidable under stress fatigue, in order to assess the ultimate sensing life of fiber Bragg grating sensors, the full scale relative error expression of stress fatigue effect is deduced from the principle of fiber Bragg grating, and the industrial instrument accuracy class is used as the limit state threshold of the sensor. And then a method based on the beam of constant strength is put forward to test the ultimate sensing life. The ultimate sensing life of fiber Bragg grating in high pile wharf structural health monitoring application is assessed by this method, the failure threshold of full scale relative error is set as 4%, 4 fiber Bragg grating sensors suffer from 132 million times alternate strain, when the fatigue number reach to 132 million times, 2 sensors reach the ultimate sensing life, 1 sensor proximity reach the limit. That is, the maximum sensing life is 132 million times. Experimental results show that the proposed method can effectively test the ultimate sensing life of stress fatigued fiber Bragg grating strain sensors in different structural health monitoring application.