The invention discloses a selective catalytic reduction denitration system suitable for medium and small ships. The system comprises an engine, a main exhaust pipe, a reactor, and an urea unit for spraying a mixture of urea water solution and compressed air to the reactor according to engine and exhaust parameters, wherein a tail exhaust pipe is arranged at the top of the reactor; the bottom of the reactor is tightly connected with an inlet exhaust pipe; an urea nozzle and a catalyst module are mounted in the reactor from bottom to top; an ash blowing device is arranged below the catalyst module; the two ends of the main exhaust pipe are respectively connected with the reactor and the engine; the urea unit is connected with the urea nozzle mounted at the bottom in the reactor; and the urea unit sprays a reducing agent needed by the reaction into the reactor through the urea nozzle mounted at the bottom in the reactor. The system adopts a compact system structure, a stable and efficient control system and the piping layout, so that the whole integration is high, the space utilization rate is further improved, and nitric oxides in ship engine exhaust gas can be effectively removed.