As the key of braked friction coefficient automatic testing system between tyre and road, the influence factors of the braked force coefficient are very important. The influence factors of braked force coefficient, including the testing speed, tyre type and model, water film thickness and the temperature of road surface etc. are studied and analysised in this paper. The water film thickness and the road surface temperature are influenced by the environment. So the analysis method and impact formulation are put up by lots of research and experiment. Finally, the correlation experiment be completed in the typical road. The results show that the method and formulation is acceptable.