The fiber-optic seismic sensor with a phase demodulator forms a complete monitoring system designed for early warning of geological hazards and structural health assessment. Based on Extrinsic Fabry-Perot Interferometry (EFPI), the system provides high precision, high sensitivity, and a wide dynamic range, enabling accurate detection of weak microseismic signals. It is passive and intrinsically safe, making it suitable for flammable and explosive environments such as underground coal mines. It can operate independently for underground microseismic monitoring or be expanded with Fabry-Perot acoustic emission probes for multi-parameter monitoring. Additionally, it is applicable to structural stability monitoring in tunnels, slopes, dams, and other engineering projects. By providing reliable data acquisition to support hazard early warning, it enhances engineering safety and disaster prevention.