Abstract: Many splitting cracks of 17-4PH steel installation base occurred in the process of wire electrical discharge machining (WEDM). The initial positions of the cracks were the same. The cause of 17-4PH steel installation base splitting cracks was analyzed by means of the scanning electron microscope, the optical microscope and the heat treatment experiment. In addition, the manufacturing processes of the installation base were also considered. The results show that the splitting crack is related to the large residual stress. The WEDM destroys the balance state of the residual internal stress and redistributes the internal stress. When the additional tensile stress produced at the front position of WEDM is greater than the tensile strength of the 17-4PH steel, the crack will be generated and rapidly expanded. The reason of the large residual internal stress in the installation base is that the residual internal stresses produced by heat treatment and machining are overlapped each other. The equilibrium distribution of residual internal stress in the installation base can be changed or reduced by resolution treatment. It can effectively prevent the splitting crack of 17-4PH steel installation base in the process of WEDM.