Moisture existed in oil liquid will lead to oxidation so as to make oil deteriorated, reduce the thickness of lubricating oil membrane and lubricity, causing oil liquid denaturalization and coalescing,becoming large molecules so as to change the viscosity of the oil liquid,form organic acid, corrode the metal surface and reduce or lose the dielectric strength. The coalescing and separating oil purifier developed and manufactured in our company has precision filtration and high efficiency dehydrating functions. It can purged the particles, emulsified water and free water of the oil liquid. For the moisture (Max. 15%) contained in oil liquid, the separating efficiency is several times and scores of time of the tradition separating methods.