A CP-CS fusion model that considers both conflict probability ( CP) and conflict severity ( CS) was pro- posed to effectively evaluate the traffic safety of existing or pre-built on-ramp merging areas on the highway． Firstly， the conflict probability model was established considering the impact of vehicle micro-running characteristics on traffic conflicts． Furthermore，the statistical data were used to integrate the conflict probability and conflict severity，so as to construct the CP-CS fusion model． Finally，based on the observation data，SSAM traffic conflict simulation analysis was carried out to verify the effectiveness of the proposed model． Moreover，a case study was introduced to reveal the evolution mechanism of risk coefficients in the merging areas，and illustrate the safety level of the merging areas under various optimization schemes． The experimental results show that the proposed model can improve the safety level of the existing or pre-built on-ramp． And there is a relatively higher traffic efficiency and safety level in the merging areas when the speed of traffic flow on the mainline is smaller than 27 m /s and the traffic flow is less than 1000 veh /h．