The invention belongs to the technical field of anticorrosive paints, and provides a waterborne rust conversion varnish and a preparation method. A waterborne composite resin emulsion system is used as a film forming matter, the coating can wet and penetrate into a dust layer of the whole steel surface through the effect of a penetrating agent, and a dust converting agent reacts with rust iron oxide formed on the surface of steel to form a stable macromolecule organic resin-iron complex, so that a dust layer binds to form a continuous closed coating layer, and the shield anti-corrosion protection effect is achieved. The coating has the certain anti-corrosion performance, and has the good matching property with other anti-corrosion coatings; by the adoption of the dust conversion varnish, construction strength and difficulty can be lowered, the labor cost is reduced, environmental pollution is reduced, construction environment is kept clean, and long-acting anti-corrosion protection is achieved for steel base materials.