KDP crystal plays an important role in inertial confinement fusion (ICF) optical system. However, how to obtain KDP crystal elements with the application requirements is still a difficult problem. The processing technology of KDP crystal flat element is studied using fly-cutting technology. Firstly, the technical principle of fly-cutting and the influence factors of the surface roughness are explained. And then, the influence factors in processing are analyzed with single variable method through a mass of processing experiments. The experimental results show that the roughness of KDP surface is affected by the diamond tool parameters, the processing parameters and the cleaning method, but the diamond tool parameters have the greatest influence on the roughness. The ultra-smooth surface can be obtained by using diamond tool with rake angle of -45 °, tool radius of 5.0mm and optimum processing parameters, and its corresponding Sa is less than 1 nm. The research results provide an effective technological scheme for the processing of KDP crystal flat elements by fly-cutting, and have extensive engineering application value.