In weigh-in-motion (WIM) system, the gross weight or the axle weight of the passing vehicle can be measured dynamically by the sensors installed in or on the pavement. One of the WIM sensors is piezoelectric sensor. Although piezoelectric sensor has limited measurement accuracy, it is widely deployed for its low cost and easy installation. To use the piezoelectric sensor, several factors have to be considered for the WIM site itself to ensure the vehicle passing over the sensor with a relative stable state. In addition, the piezoelectric sensor is seldom used for low speed measurement because of the piezoelectric material's performance. Traditional measurement method just uses the interactions between the sensor and the vehicle's tires that make the measurement inaccurate because the sensor cannot cover the whole tire patch along the driving direction. In this paper, the pavement deflection by the vehicle under measurement is introduced. New weighing method is developed for embedded piezoelectric sensors. Field tests are performed and the measurement errors are calculated based on the static weights measured from a static scale. Comparing to the traditional method, the proposed method is proved to have a higher accuracy and require less from installation site and vehicles under measurement. Furthermore, the method shows a better performance at low vehicle speed.