The invention discloses an exhaust system of a motor boat engine, which comprises an exhaust pipe, an exhaust flange and a water jacket flange; the exhaust pipe comprises a plurality of exhaust manifolds, an exhaust main, a tail pipe and an exhaust pipe flange, wherein the exhaust manifolds and the exhaust main all adopt double layer structures, and a main pipe water jacket and a manifold water jacket are formed between the inner and outer layer pipe bodies; a water inlet connector is arranged at one end of the outer pipe of the main pipe, and is communicated with the main pipe water jacket, the other end of the water inlet connector is communicated with of the cooling water outlet opening of a generalized oil pan, and the water inlet connector is taken as the main cooling water inlet opening of the exhaust system; the water jacket flange is arranged at the outlet opening of the tail pipe, and a plurality of water outlet holes are arranged on the water jacket flange; the exhaust flange is provided with a cooling water jacket, a plurality of through-holes are arranged in the position where the exhaust pipe flange corresponds to the cooling water jacket; an water inlet hole is arranged in the cooling water jacket, and communicates with the cooling water channel of the engine. The exhaust system has the advantages that the surface temperature of the exhaust pipe of the motor boat engine can be effectively reduced, the thermal protection shield of the exhaust pipe can be eliminated, and at the same time, the cabin temperature and the exhaust gas temperature are reduced, thereby guaranteeing the high reliability of the entire exhaust system.