The invention discloses a gas supply system for a marine gas engine. The gas supply system comprises an air filter arranged on a support, a mixer and an intake manifold, wherein one end of the mixer is connected with the air filter through a filter adapter, the other end of the mixer is connected with a booster, the booster is connected with an intercooler, a fuel gas inlet pipe is arranged on the side wall of the mixer, and a jet metering valve is connected to the interior of the fuel gas inlet pipe; and the intake manifold is connected with the intercooler through an intake adapter. The intake adapter comprises a first elbow, a first-segment adapter and a second-segment adapter, wherein an anti-explosion valve is arranged on the outer side of the bent wall; an intake back-fire relief valve is connected between the first-segment adapter and the first elbow; and the second-segment adapter and the first-segment adapter are parallel to the intake manifold, an electronic throttle valve is connected between the first-segment adapter and the second-segment adapter, and the second-segment adapter is connected with the intercooler through the second elbow. According to the gas supply system, fuel gas can be utilized to provide the marine engine with a fuel.