Road transport system is complex and the causes behind major and severe road traffic accidents are also complicated. Firstly, the complexity of road transport system was researched, and then,the cause factors and their complexity theory with DEMATEL algorithm on the basis of investigation and analysis to recent years’ major road traffic accidents in China were researched. The result indicates that the causes make different differences to accidents, and the key inducement is partial factor, which makes major contribution. There exist progressive hierarchical structure and complicated association among causes factors, and the essential reason to accident is ground factor, the direct reason is top factor.