In order to examine the significant factors of severity in the long and steep downgrade of mountain expressway, firstly, in-depth research was conducted on road traffic accident data of typical mountainous expressway long downhill sections located in Shaanxi Province, the Guangxi Zhuang Autonomous Region and Yunnan Province. A total of 2 559 accident data were obtained. Based on the above accident data, multiple Logit accident severity models were constructed to analyze the direct causes of traffic accidents on mountainous expressway long downhill sections. The analysis results show that speeding, illegal loading exceeding the limit, fatigue driving, improper operation, illegal parking, and vehicle braking failure of the driver in the accident have a significant impact on casualties on long and downhill sections of mountainous highways. The research results provide direction for improving the operational safety of mountainous highways and fundamentally seeking effective ways to prevent frequent accidents on built mountainous highways.