In order to accurately assess the roads' traffic safety of rainfall weather, this paper chooses the road adhesion coefficient and visibility of rainfall weather as the two indicators to study. Then, with the use of ADAMS/Car and the application of the simulation programs: differential value of lateral displacement and of the braking distance, this paper comes to the simulation results on attachment coefficient indicator and visibility indicator in wet weather. On the basis of the analysis results, this paper has established a grading program for traffic safety assessment of rainfall weather.