Abstract: The fracture reasons of 40Cr steel cylinder were analyzed by macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscope and energy spectrum analysis, microhardness test. The results show that cylinder fracture was mainly related to excessive sulfur content, sulfide aggregation, surface pit defects and uneven heat treatment. Sulfur element was easy to produce "hot brittle", aggregated strip inclusions destroyed the continuity of the matrix, so that the cylinder produced micro-cracks, stress concentration was produced at the surface pits, internal stress was produced by uneven heat treatment. Initiated cracks and surface pits continued to expand under the action of the above factors, eventually led to cylinder fracture.