KDP crystal was modified by nitrogen cold plasma to achieve high quality and high efficiency liquid film contact deliquescence polishing. Using nitrogen cold plasma to process the deliquescence polishing interface in real time, the droplet in micro vapor mist changed from droplet residence to liquid film contact on KDP crystal surface, which overcame the shortage of continuous formation of new “micro pits” caused by uneven point contact of water on the workpiece surface. By studying the material removal characteristics of KDP crystal in aqueous medium, the method to control the performance of polishing medium was obtained, and the mechanism of nitrogen cold plasma affecting the etching law of KDP crystal was revealed. The polishing mechanism of KDP crystal by nitrogen cold plasma was comprehensively evaluated by studying the friction characteristics of KDP crystal in the polishing interface, the microstructure of KDP crystal surface, Raman spectrum and the aging of hydrophilic modification of KDP crystal surface by nitrogen cold plasma. It was proved that the deliquescence of nitrogen cold plasma modified KDP crystal can further improve the surface quality of KDP crystal in the polishing process. Finally, using the optimized discharge parameters, the RMS value of the surface roughness decreased from 18.4 nm to 7.6 nm, and the PV value decreased from 109.9 nm to 61.5 nm. The lowest material removal rate was 10.14 μm/min and the highest was 91.58 μm/min. The conclusion is that using nitrogen cold plasma, KDP crystal can be quickly and non destructively treated to superhydrophilic state, which can effectively remove the micro pits caused by droplet residence, greatly improve the surface quality and reduce the scratch, providing a new idea for high quality and efficient polishing of KDP crystal.