Abstract: The piston rings used in a vessel fractured on multiple positions during use. Combined with the stress situation of the operation process, the failure causes of piston rings were analyzed by means of macroscopic morphology observation, fracture morphology observation, working surface observation and structure analysis. The results show that the microstructure of the piston ring material couldn’t meet the requirement of the standard, and the inner wall of the cylinder jacket was not repaired during lifting the cylinder. The apparent collision and extrusion deformation on fracture surface was the main reason of the fracture of the piston rings.