The flat cover type leakage-proof diesel oil tank aims to overcome the defects of an existing diesel oil tank and is not prone to corrosion and light in weight. The oil tank comprises a tank body, anupper cover, an oil filling port and an oil tank accessory, the oil tank accessory is fixed on the upper cover, the tank body is formed by stretching an electro-galvanized sheet, the upper cover is aflat plate, the edge of the flat plate is welded and fixed with the upper cover, the oil filling port and the upper cover are integrally formed, and the lower end face of the oil filling port is lowerthan a welding seam. The electro-galvanized sheet has an anti-corrosion effect; the lower end surface of the oil filler is lower than a welding seam between the tank and the upper cover; the oil level is controlled below the lower end surface of the oil filling port during oil filling; therefore, the highest point of the oil level of the oil tank is below the lower end face of the oil filling port, namely the highest point of the oil level of the oil tank is below the weld joint between the tank body and the upper cover, diesel oil cannot soak the weld joint between the tank body and the upper cover, oil tank accessories are fixed on the upper cover, the weld joint between the oil tank accessories and the upper cover cannot be soaked by the diesel oil, and the oil.