With the wide application of ZnS crystal optical elements in infrared optical system, the surface quality of ZnS crystal optical elements is more and more demanding, but due to the brittleness of the material, it is difficult to obtain high quality surface roughness. In order to obtain high quality surface of ZnS crystal, the principle of cutting and fly-cutting based on single point diamond lathe was introduced, as well as the factors affecting surface roughness. And then the influence of different parameters of diamond tool and different processing parameters on the surface roughness of ZnS flat element was studied by single variable method through process experiment. The quality of the machined surface was tested by microscope and white light interferometer, and the processing parameters were optimized by feedback. Finally, based on the optimal processing parameters, high quality ZnS flat optical elements with surface roughnessSa of about 1 nm were obtained in the both different methods. The results provide technical support for the development of high quality optical elements of ZnS crystal and have good engineering application value.