The invention relates to a water-based ant-rusting agent for an oil tank of an automobile and a preparation method thereof. The anti-rusting agent comprises the following components in part by weight: 60 to 70 parts of epoxy acrylate resin, 80 to 100 parts of aqueous fluorocarbon emulsion, 8 to 10 parts of methanol, 3 to 5 parts of butanol and 3 to 5 parts of water. The preparation method comprises the following steps: sequentially adding the methanol, the butanol and the water into the epoxy acrylate resin; stirring and dissolving the mixture; and adding the aqueous fluorocarbon emulsion into the dissolved mixture and fully stirring the mixture to obtain the water-based anti-rusting agent for the oil tank of the automobile. The anti-rusting agent has economical composition, excellent anti-rusting effect and simple process of the preparation method. The anti-rusting agent can also be used on an ordinary steel plate to meet the anti-corrosion requirement of the oil tank of the automobile, so the production of the oil tank by using the ordinary steel plate becomes possible. Compared with the oil tank made of galvanized sheets, the steel plate oil tank produced by the ordinary steel plate greatly lowers production cost. The anti-rusting agent is easy to store and is preferably kept at a cool, dry and ventilating place at the temperature of above 5 DEG C without being directly exposed in the sun. Under a normal storage environment, the anti-rusting period can be over 60 months.