Owing to the complex surface profile of diffractive optical elements (DOE), it is extremely difficult to further improve the obtained surface roughness of DOE by polishing without damaging its form accuracy. This research aimed to design and develop a non-contact precision polishing system, based on the float polishing method, to improving the surface finish of DOE and, in the same time, to preserve its form accuracy. Polishing parameters such as relative speed between the workpiece and the polishing molds, abrasive size, polishing gap, different kinds of the polishing molds were correlated to the obtained surface roughness, form accuracy and material removal rate in the present study. The results showed that the system developed in this research demonstrated good surface roughness improve rate on polishing the DOE without surrendering too much form accuracy.