In order to understand the electro-mechanical characteristics of the piezoelectric sensor in weigh-in-motion system and theoretically support the data post-processing, this paper established a 3D solid finite element model of beam type piezoelectric sensor of a weigh-in-motion system, which was embedded in the road. The vehicles ware simulated as moving concentrated load. The signals of the output voltage of the piezoelectric sensor were then obtained in different speed and weight of vehicles. The results showed that the output signal of the piezoelectric sensor could be used to extract the effective information of the axle-weight and speed of the vehicle.