Abstract: Aero-engine hydraulic actuating cylinders of 1Cr11Ni2W2MoV heat-resisting martensitic stainless steel by electron beam welding were found to leak oil after welding bulge testing or factory testing. In order to find out the failure cause, macro and micro observation, metallographic examination, and hardness testing were carried out. The material, welding method and quality were investigated. The results show that the welding cracks of the failed actuating cylinders were attributed to unstable processing parameters during welding. The cracks could be avoided by using other equipment with stable processing parameters.