Abstract: In this paper, crack of the main shaft of No.2 unit in a hydropower station is taken as the research object, and causes of the crack are studied by the methods of field investigation, nondestructive testing and physical and chemical testing. It is concluded that the fatigue crack is caused by the comprehensive effects of stress concentration, installation process, operation conditions, etc. According to the actual situation of the production site, the optimization and improvement measures of operation and maintenance are put forward.