In order to reveal the constant-fatigue fracture form and mechanism of the welded cross plate-hollow sphere joints (WCPHSJs) and establish its formula, the WCPHSJs were fatigue tested. A total of 19 specimens were tested under constant amplitude fatigue loads using a specially designed test rig. The joint was analyzed statically by the finite element analysis ( FEA), and metallographic analysis of fatigue fracture was done by the electron scanning microscope. Numerical simulation and experimental results show that the hot-spot of WCPHSJ lies at the weld toe location where severe stress is concentrated. Fatigue cracks initiate at the weld toe and then propagate circumferentially around the sphere with a diameter equivalent to the width of the cross plate up to the fatigue fracture. The initial welding defects and constructional detail constitute the main factor of fatigue failure. The S-N curves for the joints were developed through a linear regression analysis of fatigue data. A formula for calculating constant amplitude fatigue, based on the concept of the hot spot stress amplitude, is proposed.