As the key part of nuclear pressure vessels, the water chamber head has the features of large size, complex shape and difficult processability. The processes of conventional machining method are too decentralized, the cumulative error is large, and the processing cycle is long. In order to meet the efficiency and high-precision machining requirements of the nuclear power water chamber head, a design scheme of special turn-milling machining center and B-axis components is proposed. The part characteristics and machining difficulties of the water chamber head are analyzed in the paper. The machining areas and processes are subdivided, the numerical control technological process is presented. In order to complete the finish machining of the water chamber's inner and outer surfaces and its inclined holes and sloping faces, the movable gantry milling machining center with pendulant ram is designed. According to the head processing requirements, three design schemes of B-axis components assembly, single row cylindrical roller bearing type, double row tapered roller bearing type and the central axis unloading type, are proposed. Through the theory analysis and statics and modal comparative analysis, the central axis unloading type is adopted as the optimal proposal in B-axis component design. Simulation and experimental results show that the turn-milling machining center based on the unloading B-axis assembly can satisfy the precision requirement of the water chamber head.