The invention discloses a fuel gas adjusting device of a gas engine. The fuel gas adjusting device of the gas engine is used for the technical field of gas engines. The fuel gas adjusting device of the gas engine comprises a speed adjusting shaft, a speed adjusting shaft drive mechanism which can drive the speed adjusting shaft to rotate and fuel gas control valves which are arranged correspondingto air cylinders of the gas engine. The speed adjusting shaft is transversely arranged on the gas engine through a speed adjusting shaft base and passes through the air cylinders of the gas engine from one side. The speed adjusting shaft is provided with a first rocker arm assembly which can drive the corresponding fuel gas control valves. The first rocker arm assembly comprises a first rocker arm arranged on the speed adjusting shaft and a first connection rod connected between the first rocker arm and the fuel gas control valves. When the fuel gas adjusting device of the gas engine works, the speed adjusting shaft drive mechanism drives the speed adjusting shaft to rotate, the speed adjusting shaft drives the fuel gas control valves through the first rocker arm assembly, and therefore the amount of fuel gas entering each air cylinder can be controlled and adjusted, a mechanical transmission control manner is adopted, the manufacturing cost of the engine can be saved, and meanwhile the debugging difficulty and the using difficulty can be lowered.