The Kistler company has invented piezoelectric dynamic weighing sensors which are applied to determine dynamically axle load, speed and gross weight of the vehicle based on piezoelectric effect of quartz crystal. They mainly consist of load-carrying beams and sensitive elements. Focusing on the control of preload in design and fabrication, finite element method (FEM) is applied with ANSYS software to optimize structural sizes. The influence curve of the structural dimensions of load-carrying beam on the preload is analyzed. With the piezoelectric coupling analysis function, the preload influence on the sensor sensitivity is researched. FEM simulation results show that the critical dimensions of load-carrying beam as well as the deformation and wall thickness of cavity have significant impact on the preload. Further studies show that the sensor has the highest sensitivity with the reasonable deformation and wall thickness of the cavity. In this paper, a sensor with range of 150 kN has the highest sensitivity at the wall thickness of 2 mm and the deformation of 0.14 mm.