The present invention provides an annular cavity ring-down spectroscopy-based strain sensor and a strain measurement method. According to the annular cavity ring-down spectroscopy-based strain sensor and the strain measurement method, continuous light emitted by an ASE light source passes through an optical polarizer and a photoelectric modulator so as to be converted into pulse light; the pulse light is decayed circularly several times in an annular ring-down cavity; in each circulation, only a small part of the signal light is outputted through the first output end of a second coupler, and then is detected by a photoelectric detector, and the remaining part of the signal light continues to be decayed in the annular cavity; when a sensing fiber grating is strained, the relative positions of the spectrum of the sensing fiber grating and the spectrum of an auxiliary fiber grating change, which results in the change of the loss of the annular ring-down cavity, and as a result, the ring-down time of the pulse signals changes, and therefore, the strain of the sensing fiber grating can be obtained through detecting the ring-down time of the pulse signals; and since the temperature responses of the sensing fiber grating and the auxiliary fiber grating are identical, temperature change will not change the loss of the annular ring-down cavity and the ring-down time of the optical pulse signals, and therefore, the sensor can realize automatic temperature compensation.