In this paper, the vehicle mounted laser pavement flatness detection system based on the highway comprehensive detection vehicle is adopted. Through experimental research, it is found that there are abnormal fluctuations in the vertical section elevation and the corresponding international roughness index IRI test results under continuous longitudinal slope and small radius circular curve sections. The main reason is that the vehicle posture is tilted back in the continuous uphill stage, and the vehicle posture is dive in the continuous downhill stage. When turning left, the vehicle posture is tilted right, and when turning right, the vehicle posture is tilted left. The pitch and roll of the vehicle posture will offset together with the laser ranging sensor. As a result, the output of section elevation data is abnormal, and the international flatness index IRI test results are distorted. Testing results show that the vehicle mounted laser pavement flatness detection system based on laser ranging and uniaxial acceleration inertia correction technology is not applicable to the complex linear sections of low-grade highway, and the corresponding laser ranging correction technology needs to be further studied.