Polishing is an important final machining process in manufacturing. For robotic polishing, active compliance control is the most frequently used approach to control the contact force between the end-effector and workpiece. However, it usually exhibits the problem of force overshoot at the start of contact, with poor force accuracy normally larger than 1 N. This paper proposes the concept design of a novel end-effector based on constant-force mechanism for robotic polishing. This design is the first constant-force mechanism based robotic end-effector for use in polishing experiment. An industrial robot is used to position the end-effector and the end-effector regulates the contact force passively. 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, producing the consistency for the workpiece's surface quality. Moreover, the property of the constant force ensures more accurate contact force without using a complex controller. Design, modeling, and simulation study have been performed to demonstrate the proposed idea. A prototype is fabricated for experimental testing. The end-effector is adopted to polish rusty steel, and the recorded contact force signal during polishing demonstrates the effectiveness of the constant-force mechanism in counteracting the force overshoot and improving the force accuracy. Results indicate that the polished surface is extremely uniform with steady contact force regulated by the constant-force mechanism.