Abstract: Deflection monitoring is an important part of bridge health monitoring. A high-precision deflection measurement system based on laser-PSD (Position Sensitive Detectors) for simply supported girder bridges is proposed. The system adopts a laser reflection method to amplify the deflection of simply supported girder bridges and realize static and dynamic monitoring of bridge deflection with micrometer-level accuracy. The composition and working principle of the system are firstly introduced, which can amplify the bridge mid-span deflection by more than 7 times through the laser reflection principle to achieve high-precision deflection measurement; Through theoretical derivation and error analysis, the influence of the placement of the laser transceiver and reflector on the deflection amplification factor is discussed; The effectiveness of the system is verified through experiments. The system can accurately measure the mid-span deflection of a simply supported girder bridge with an accuracy above 1 μm for the measurement range of millimeter level. It can perform dynamic deflection measurements so as to identify dynamic parameters such as the fundamental periods of the bridges. The laser-PSD measurement system proposed in this paper features high accuracy, fast response, low price, and easy-to-implement for long-term dynamic monitoring. This study is expected to provide a reference for deflection health monitoring of small and medium span simply supported girder bridges.