With the advancement of hydraulic transmission systems toward high-pressure and high-speed operating conditions, the reliability of the plunger pump—a core power transmission component—faces significant technical challenges. This study focuses on a rotor cylinder block with a copper-steel bimetallic structure, manufactured through inter-facial metallurgical bonding technology for dissimilar metal integration. During operation, this component simultaneously forms two critical friction pairs: the plunger-cylinder bore friction pair and the cylinder block-port plate end face friction pair. As the core load-bearing element restricting pump fatigue life, this study focused on the ultra-high cycle fatigue performance of a cylinder block structure manufactured via a bimetallic process. The fatigue life prediction for the cylinder block structure is preliminary achieved, with prediction results closely matching actual service life of the plunger pump. This research represents a significant exploratory effort. Further studies on the crack propagation mechanisms based on these findings could provide theoretical support for the design of high-reliability plunger pumps.