The invention discloses a ship, a propulsion system and a braking device and method thereof. A braking method of a propulsion shaft comprises the following steps that after it is detected that a mainmachine is disengaged from a gear box, the rotation speed of the propulsion shaft is detected; when it is detected that the speed of the propulsion shaft is decreased to the first preset rotation speed, a first braking assembly is started to be in friction fit with a brake disc; when it is detected that the rotation speed of the propulsion shaft is decreased from the first preset rotation speed tothe second preset rotation speed, a second braking assembly is started to be in locking fit with the brake disc, and the propulsion shaft is locked. According to the ship, the propulsion system and the braking device and method thereof, it can be ensured that the propulsion shaft is not damaged in the braking process, potential safety hazards are completely eradicated, and the ship using requirements at the present stage can be met.