A ring-cavity fiber laser based fiber Bragg grating (FBG) sensor system, in which two FBGs are used, is reported in this paper. One of the employed gratings, acted as the end mirror, is sensor element. Another is used as a tunable filter controlled by a piezoelectric stack, which is driven by an electrical sine wave signal. The measured strain applied on the sensor axially can be judged by observing the interval of the output laser between two notches within a driving period in time-domain. Experiment demonstrated that the sensing sensitivity and the resolution of this system are 88.152 με/ms and 1.8 με respectively, within the work range of from -140 με to 720 με. The perturbation of the environmental temperature was proved not to affect the measured results theoretically. It was verified by experiments.