In response to the problem of low wavelength scanning resolution of side hole fiber Bragg grating sensors in high-temperature and high-pressure annular mirrors underground, the traditional sensing optical path for frequency demodulation is improved, and a laser frequency based side hole fiber Bragg grating pressure sensing system suitable for underground annular mirrors is designed, which can perform long-distance sensing underground and effectively improve its resolution. In addition, combined with self-injection locking technology, it separates the sensor from the fiber laser unlike the traditional beat frequency demodulation optical path that uses lasers as sensors, making it easy to be applied in underground environments. The system uses a linearly chirped fiber Bragg grating as a dispersion compensation unit and a reflector on one side of the laser, transforming the wavelength change caused by pressure on the fiber Bragg grating into the cavity length change of the resonant cavity, thereby improving the detection sensitivity of the system. Moreover, the structure and measurement principle of the method used are explained, and the measurement results and accuracy are analyzed and calculated. The experimental results show that the system has high sensitivity and measurement accuracy with a pressure sensitivity of 0.12553 MHz/MPa. The proposed method has the advantages of a simple demodulation system, good stability, and adjustable sensing distance.