The characteristics of traffic accidents is analysed in order to determine the mainline alignment index in the diamond interchange diverging areas. The recognition sight distance of the diamond interchange diverging areas by combining the automobile theory, traffic psychology and ergonomics,and simultaneously are acquired. Based on the VB software and the setting conditions of the lateral clear distance of the expressway, the relation model between the indicators of the mainline plane and vertical section and the recognition sight distance in the diamond interchange diverging areas is set up. The values of the circular curve radius and the convex vertical curve radius of the mainline which meets the requirements of the recognition sight distance in the diamond interchange diverging areas is obtained. The analysis results show that it is necessary to analysis to the recognition sight distance of the diamond interchange diverging areas to be constructed and check the alignment index of the mainline with the recognition sight distance, for the constructed diamond interchange diverging areas, it is necessary to strengthen the safety precautions so as to guarantee the traffic safety of vehicles driving on the mainline and off the mainline.