The safety of roads, bridges and other transport infrastructures subjected to heavy traffic and overloaded vehicles has been widely concerned throughout the world. Accurate vehicle information collection is of vital importance to the road service and structural health monitoring. The newly developed Bridge Weigh-in-motion (BWIM) system is an efficient and economical technology to obtain vehicle information. BWIM system uses an instrumented bridge as a large scale to accurately identify vehicle information such as vehicle speed, axle numbers, axle spacing, and vehicle weights without interrupting traffic. In this study, the field tests were conducted on a continuous box girder bridge in Qingyuan, Guangdong, China. The fast Fourier transform (FFT) method, which can significantly purify the original data from transducers, has been employed to improve the accuracy of axle detection, weighing, and counting. The averaged influence lines from different lanes in the field tests were selected for axle weight identification, and the accuracy of identified axle weights was compared and evaluated.