The first auxiliary dam of Huojia Reservoir is a dual arch dam.It was built and put into operation in 1980.Shortly after operation，three vertical cracks appeared on the downstream surface of the first arch dam.In order to analyze the causes of cracks，the finite element analysis method is used to reveal the causes of cracks in arch dams by analyzing the displacement and stress of arch dams under different load combinations.The research shows that the dam body is too thin，the stiffness of the arch ring and arch beam of the dam body is insufficient，and the radial reciprocating deformation of the dam body occurs when the temperature rises and falls，both of which are the main causes of cracks in a heavy arch dam.