In order to know the operation condition of bridges when they’re under the action of overweight vehicles, it’s necessary to collect the testing information of bridges under the overloaded condition. Based on the high-speed demodulation instrument, the optical fiber grating (FBG) sensor system can realize the continuous collection for the dynamic strain of structure. According to the analysis of the collected data, auto-sorting and recognition for overweight vehicles can be realized. The introductive system has been applied to the health monitoring system of no.2 Wuhan Bridge over Yangtze River (N2WB). The result showed that not only it’s able to recognize whether or not there was an overweight vehicle passing the bridge, but also it could auto-sort the grade of overweight vehicles, which was an effective overweight vehicle monitoring tool for the existing large span bridges.