The invention discloses a coating large-area continuous deposition and surface modifying method and belongs to the technical field of material surfaces. According to the method, cathode plasma electrolysis adopts electrolytic cell solutions with heavy organic liquid on the lower layer and an aqueous solution electrolyte on the upper layer. By controlling an electrode to be immersed into the electrolyte from the liquid surface, cathode metal is subjected to coating deposition or surface modifying treatment through plasma discharge in the aqueous solution electrolyte layer. By adjusting the area ratio of an anode to a cathode and applying the direct voltage or the pulse voltage to the cathode and the anode of an electrolytic cell, and in electrolytes of different components, oxides, carbides, borides and composite ceramic coatings can be deposited on the surface of the metal, and various metals such as nickel, copper, chromium and cadmium or metal composite coatings can be also deposited; and meanwhile, by introducing carbides and nitrides into the solutions, plasma nitriding, plasma carburizing or plasma carbonitriding of the metal cathode can be further achieved, besides, the material surface can be cleaned, oxides can be removed, and nanocrystallization of the material surface can be achieved.