It is crucial for serial robots with redundant degrees-of-freedom to accomplish complex tasks with high pose precision of the end-effector. However, due to the cumulative errors in each joint actuator, it is always challenging to guarantee high pose precision of the end-effector. To address this issue, this study proposes a virtual-constraints-based end-effector pose compensator. The actual angles of specific joints are treated as known variables, which are called virtual constraints. The virtual constraints, combined with the expected pose of the end-effector, yield compensation angles for the other joints. The compensation angles are applied to the expected angles of other joints, eliminating the end-effector pose errors caused by the joints treated as virtual constraints. Finally, a typical planar 3-degree-of-freedom serial robot (the leg hydraulic drive system for legged robots) is utilized for experimental validation. The experimental results demonstrate that, with the application of the compensator, the maximum reduction in position error of the foot end exceeds 75%. The compensation strategy further improves the end-effector pose precision based on various high-precision position control methods for the joint actuators. In addition, this study introduces a novel idea and provides experimental evidence for compensating the end-effector pose error of redundant serial robots. Display omitted