Abstract: Cracks are found with a certain batch of engineering machinery-purposed hydraulic cylinders during testing process.Accordingly the causes are analyzed.As a result it is concluded that there exist some welding slag on the fracture of the crack source area,and the interlayer non-fusion defect in the welding part which together deteriorate the bonding strength of the welding part,and moreover there are abnormal widmanite,martensite,and bainite structures in the welding heat-affected zone(HAZ),which work as a whole to lead to the consequence that during pressure test process the cracks develop in the said defective part of the cylinder,and then expand into the cylinder case proper.Therefore it is recommended to strictly implement the welding process procedures during the welding process,improve the welding quality,and eliminate welding defects,so as to provide safety guarantees for the subsequent testing and use of the cylinder.