Aiming at the problems of diversified maintenance operations, numerous risk factors, and frequent safety accidents in hydropower station units, the safety factors affecting the maintenance operations of hydropower station units are classified through literature mining and expert interviews.Based on the DEMATEL-ISM method, a multi-level hierarchical ISM model of safety accident causes was constructed to determine the level of accident causes. By analyzing the centrality and weight of accident causes, mining the key causal factors and deriving the path weights of the accident causal chain, the key causal chain was determined. The model was used to analyze the key causal chain of a hydropower station as an example. The results show that inadequate safety inspections are the key cause factor, and the key causation chain is inadequate safety inspections→illegal operations→equipment problems→dusts, paints, and other harmful substances.