Abstract: A passive hydrological monitoring system for coal mine goaf based on fiber optic sensors is designed to address the problem of the existing mining hydrological monitoring system being unable to directly connect electrical signals to the goaf during the process of mining,which results in the inability to monitor the water level in the goaf in real time. The measurement principle of water level sensor based on Bragg grating and the demodulation principle based on wavelength division multiplexing are discussed,and the overall system scheme and hardware scheme of the demodulator are designed. The monitoring system was deployed on site before the mining of the I030902 working face in Limin Coal Mine to monitor the water level in the goaf in real-time during the mining process The system operated stably and reliably during the monitoring process,effectively saving the amount of water exploration and improving the accuracy of the calculation of water accumulation in the goaf.