Abstract: To find out the causes for the bursting of the 45 steel hydraulic cylinder during hydrostatic test,analyses are conducted on the results of relevant tests as made to the cylinder,including macroscopic analysis,the metallurgical analysis,the SEM analysis,the EDS analysis,and the mechanical and chemical property testing etc.It is identified via the analyses that the main reason for the said busting is the welding cracks which are caused by absence of stress-relief annealing treatment,or insufficient stress-relief annealing treatment.Accordingly it is suggested to take such actions as strict control of the welding process,adding necessary after-welding NDT inspections and ensuring the as-normalized furnishing state of the steel tube.