The basic working principle is: it moves forward under the traction of the tractor, and there is a 0.5-1mm water film between the friction wheel and the ground. Due to the speed difference between the friction wheel and the running wheel, the test wheel produces a slip rate of 12-17%, which will generate a longitudinal force perpendicular to the driving direction. This longitudinal force serves as a characteristic value representing the friction coefficient of the road surface, and is determined by high-precision. The force sensor measures and transmits it to the computer for processing; the road mileage is measured by a high-resolution encoder, and the microcomputer gives the corresponding table and curve of the road position and friction coefficient. After data processing by the software, the measurement results of the longitudinal force friction coefficient correspond to the road station number, and the longitudinal force coefficient for each section (5 meters, 10 meters, 20 meters, 100 meters, and 1 kilometers can be selected) is given. The vehicle speed is obtained from the encoder, and the sprinkler device automatically adjusts the water output according to the vehicle speed so that the thickness of the water film between the test wheel and the road surface is 0.5~1mm.