A ship waste gas denitration system comprises a waste gas direct exhaust pipeline connected with a gas outlet of a waste gas collecting tank through a first valve, a waste gas denitration pipeline connected with the gas outlet of the waste gas collecting tank through a second valve and a denitration reactor connected in the waste gas denitration pipeline, wherein the denitration reactor comprises a urea evaporation chamber, a mixer and a reacting chamber. The urea evaporation chamber is provided with a urea solution spray gun. The mixer is connected with the urea evaporation chamber and the reacting chamber. When the first valve is opened and the second valve is closed, waste gas is exhausted through the waste gas direct exhaust pipeline; and when the first valve is closed and the second valve is opened, the waste gas enters the urea evaporation chamber of the denitration reactor through the waste gas denitration pipeline, then is mixed with urea, enters the reactor through the mixer, and is subjected to denitration and then exhausted. According to the ship waste gas denitration system, the urea evaporation chamber, the mixer and the reacting chamber are combined in the denitration reactor, so that the structure is more compact.