Abstract: Addressing the fact that the finished 45 steel hydraulic cylinder pipe as made with the process of colddrawing plus scraping-rolling develops through-cracking after being used for 11 hours under about 20 MPa pressure, the reason for the said cracking is analyzed by means of studying such aspects as fracture morphology, composition of the corrosive product, mechanical property and microstructure, etc.It is regarded via the analysis that affected by such comprehensive factors as rather low temperature for stress-relief annealing process and reticular ferrite microstructure, etc., both the plasticity and toughness of the 45 steel hydraulic cylinder pipe are reduced, whereas its brittleness is increased, which leads to reduction of the pressure resistance as resulting in the said cracking.