The invention relates to an oil saving method by reducing the combustible mixture concentration of a gasoline engine, which can reduce the combustible mixture concentration to optimum oil saving valueof 1 to 20.3 by reducing the oil level of a float chamber by adjusting the float of the carburetor float chamber, reducing the diameter of a measuring orifice, controlling the spraying flow of an oilspraying tube and adjusting the opening amount of an air throttle. In order to overcome the deficiency that the use of thin combustible mixture is likely to reduce the power of the gasoline engine, aspark advance control device is adjusted to start the ignition when a piston is 13mm to 14mm from an upper dead point, thereby increasing crankshaft speed, keeping the original power from changing and ensuring the normal operation of the gasoline engine at the concentration of the thin combustible mixture. As the combustible oil mixture concentration of the gasoline engine is reduced to 1 to 20.3, the complete oil combustion of the combustible mixture is ensured, great oil saving purpose is achieved, environmental pollution is reduced, and economic benefit is improved.