<p>The surface roughness is the pivotal factor to affect the high resolution and accuracy of aluminum mirror in the ultraprecise optical system, the typical electroless Ni-P amorphous coating is widely applied as the polishing layer on aluminum mirror to obtain the ultralow surface roughness after ultra-precision machining, while it still faces the inevitable formation of hole, and its high hardness is detrimental to the tool wear. Herein, a novel electroplated Ni-S amorphous coating is proposed to serve as the polishing layer on aluminum mirror, it possesses the lower hardness of 408 HV_0.05, and the homogeneous distribution of hardness and microstructure ensures the smaller turning mark and surface roughness after the ultra-precision machining. Thus, this coating exhibits the superior machinability for aluminum mirror and can be potentially used as the polishing layer to replace the existing electroless Ni-P coating.</p>