Abstract: In order to solve the problem that the position test is easily interfered in the sensitive environment, an anti-theft system based on reflective optical fiber sensing is proposed. The system uses a T-type optical fiber probe as a passive sensing probe, and the reflective accessories are installed on the surface of the security enclosure to provide security early warning through real-time testing of position offsets. The detection module fixes the optical fiber probe and the reflection accessories in the security position by pre-testing the optimal reflection position. On the basis of acquiring the laser echo signal, the field intensity distribution function with respect to the position is calculated. In the calibration experiment, the test position was increased from 0 to 5.0 mm, the maximum response optical power was 18.76 μW, and the maximum variance was 0.124. When the curve exceeded 3.0 mm, the monotonicity of the curve was good, which meets the system design requirements. In the comparison experiments of different interference types, the influences of humidity, temperature and vibration on the system test results were compared. The experimental results show that whether the optical power fluctuation of the interference term is linear or non-linear, the fluctuation range is less than <4 μW, and the fluctuation degree is greater than 18 μW under the real artificial opening condition. At the same time, the displacement offset calculated by the system inversion is also changed significantly, which is more than 8 times larger than the average displacement deviation of the interference term. The system has the advantages of passive sensing end, high sensitivity and good real-time performance, and can be effectively applied to security monitoring in sensitive environments.