This paper proposes the design of a new robot end-effector based on compliant constant-force mechanism for robot-assisted manufacturing, such as polishing. One uniqueness of the proposed end-effector lies in that it offers a constant contact force without using a force sensor and controller. An industrial robot is adopted to position the end-effector and the end-effector regulates the contact force passively. When the end-effector contacts the workpiece, the constant-force motion range acts as a buffer to counteract the excessive displacement caused by inertia. As a result, there is no force overshoot, protecting the consistency of the workpiece. The analytical model of the constant-force mechanism is deduced and the structural parameters are optimized to maximize the constant-force motion range under other constraints. For experimental testing, a prototype of the constant-force end-effector has been fabricated. The mechanism exhibits constant-force tendency with the force varying from 3.4 to 4.2 N between 0.7 and 1.7 mm. Experimental results verify the effectiveness of the presented constant-force end-effector mechanism.