In this paper, a hybrid serial–parallel forging manipulator was put forward, which is composed of a planar five-bar mechanism and a two-limb parallel mechanism in series. Forging manipulator’s DOF properties and motion principle were analyzed by means of screw theory. The position solution of the main motion mechanism of manipulator was derived in the grasping stage, which was used to solve input displacements of lifting and pitching hydraulic cylinders depending on the requirements of forged piece’s lifting height and pitching angle. Dimensional optimization of the manipulator’s main motion mechanism is carried out based on position equations, resulting in that the gripper carrier’s lifting motion was decoupled from the horizontal motion. The analytical expression between driving forces of each hydraulic cylinder and external load was derived by solving the first-order derivative of position equations and using principle of virtual work. The results showed that for the parallel-link manipulator, when gripper carrier is horizontal, driving force of lifting hydraulic cylinder is only related to forged piece’s gravity but not to gravitational moment. The mechanism of this phenomenon was also further studied. Experimental model with ratio 1:20 of forging manipulator’s lifting mechanism was developed, and kinematic and static experiments were conducted on it to verify the dimensional optimization results and the conclusion was obtained by the statics.