Abstract: The diaphragm fracture in a diaphragm compressor occurred frequently in the edge and middle regions above the outer circular groove of a perforated plate. The circumferential crack indicated that exceeded radial stress was the main reason of failure. Due to the oil shortage in an oil cylinder caused by the unstable operation of a plunger pump, the diaphragm would cling to the perforated plate while the piston reached the bottom dead center. The part of the diaphragm above the groove would endure radial stresses caused by the large deflection deformation while the diaphragm clung to the perforated plate, and by the additional deformation in the groove region, respectively. In this paper, two kinds of radial stresses were calculated with the theory of a thin plate with large and small deflections, respectively. When the suction pressure was 1.7MPa, the combined radial stresses in the two edges and the middle region of the outer groove were 272.6MPa, 275.7MPa and 220.4MPa, which all exceeded an allowable radial stress, 170MPa. Meanwhile, the radial stresses caused by additional deformation were 140.1MPa, 138.2MPa and 107.4MPa. Thus, the fatigue life of a diaphragm was decreased by its additional deformation.