A high-pressure SCR system with venting and pressure-stabilizing for marine diesel engine, comprising an SCR reactor, a gas intake pipeline, an exhaust pipeline, a bypass pipeline, a pneumatic pipeline, first and second auxiliary pipelines When denitrification treatment is needed, the exhaust gas can enter from a flue gas inlet, sequentially pass through the gas intake pipeline, the SCR reactor, and the exhaust pipeline, and be discharged from a flue gas outlet. When denitrification treatment is not needed, the exhaust gas can enter the bypass pipeline from the flue gas inlet and be discharged from the flue gas outlet, the exhaust gas in the SCR reactor and exhaust pipeline being pushed by compressed air entering from the first and second auxiliary pipelines to be discharged from the flue gas outlet. Also provided is a ship.