The photothermal effect can induce the frequency fluctuations of π-phase-shifted fiber Bragg grating (PSFBG), so the incident laser is attenuated to the single photon level in order to eliminate the photothermal effect. The intensity of the incident single photon is modulated, and then the transistor-transistor logics (TTL) signal pulses of photoelectric conversion are directly demodulated with the lock-in amplifier. Thereby the environmental temperature is measured by using the PSFBG. The precision for temperature measurement is up to 0.14 ℃.