Overload and oversize transport not only cause serious damage to highways, but also bring about irreparable traffic accidents. WIM technology can control the overload and oversize of vehicles and provide a reliable basis for the heavy truck toll collection. The main existing WIM equipment includes bending-plate (weighing part wheel load), single-scale-platform (weighing load of single-axle), double-scale-platform (weighing load of single-axle), axis-group-scale (weighing weight of axisgroup), and vehicle-scale (weighing total weight of vehicle). They have many advantages and disadvantages in cost, installation, maintenance, traffic efficiency and accuracy of WIM. The WIM accuracy is one of the key indicators considered in application of technology governance, toll by weight, and traffic investigation. The carrier length of WIM system increases gradually with the increase of WIM accuracy, which leads to the construction cost and time increase also. In order to ensure the WIM accuracy, considering the economy of the carrier length, based on the vibration characteristics of the vehicle and the scale, the mathematical model of the WIM signal is established by means of moving average filtering and B spline least square method, and the vibration disturbance of low frequency dynamic load is effectively eliminated. The test result shows that (1) the weight error is less than 0.5% if test vehicles pass through the 0.8 m and 1.6 m long scales in the speed range of $0–5\mathrm{km}/h$ and $0–12\mathrm{km}/h$, which meets the national standard about Class 1 scale; (2) if test vehicles pass through the 2.6 m and 3 m long scales with the theoretical limit speed of $20.7\mathrm{km}/h$ and $24.3\mathrm{km}/h$, the statistical weight error is less than 0.5%. The research not only provided a theoretical basis for designing the length of the scale, but also has important reference value for improvement of the WIM accuracy of the existing installed single scale and double scales.