This article presents the design and testing of a novel passive polishing end-effector with adjustable constant force and wide operating angle, which enables adjusting the polishing effect and maintaining the constant contact force for polishing surfaces of large curvature. The constant-force mechanism is obtained by composing a positive-stiffness mechanism and a negative-stiffness mechanism. The magnitude of the constant force can be regulated by adjusting the preloading displacement of the positive-stiffness mechanism. The constant force is provided at a wide tilted angle of the end-effector because the connected counterweight compensates for the gravity effect. The working principle and mechanism design of the proposed polishing end-effector are presented. Based on finite-element analysis simulation, an optimization study is carried out to determine the structural parameters of the constant-force mechanism with expected performances. A prototype of a polishing end-effector is fabricated for experimental tests to evaluate its performance and verify the proposed concept design. The results indicate that the reported polishing end-effector exhibits adjustable constant force and wide operating angle for practical applications.