The invention relates to an automobile body composite coating layer which is characterized in that a first layer of the composite coating layer is a phosphorus-free conversion film, and the film thickness is 0.2-0.5 mu m; a second layer is a thin-film high-throwing power electrophoretic coating layer, and the film thickness is 15-18 mu m; a third layer is a water-based plain-colored paint coating layer, and the film thickness is 35-40 mu m; the total film thickness is 50.2-58.5 mu m; a finish paint of the water-based plain-colored paint coating layer consists of a coloring pigment occupying 18-22 percent of the total percent, emulsified resin occupying 14.5-15.5 percent of the total percent, hydrophilic water-soluble resin occupying 5 percent of the total percent and melamine resin occupying 10 percent of the total percent; on the premise of ensuring the quality of the composite coating layer, an advanced phosphorus-free pre-treatment process and a water-based plain-colored finish paint varnish-free process are adopted, so that the coating process is reduced, and a coating process is shortened; with the adoption of an advanced pre-treatment material, a thin-film high-throwing power electrophoretic coating and the water-based single-coated plain-colored finish paint, the film thickness of the composite coating layer is reduced, so that the production line investment is reduced, the coating consumption is reduced, the VOC discharge quantity is reduced, the comprehensive coating cost is reduced, and green and environment-friendly coating is realized.