There are great challenges to the air environment design of some ship cabins with large space, high occupant density and complex structure. The traditional ventilation method cannot meet the requirement. The application of the displacement ventilation system for a ship cabin is then researched by adopting the computational fluid dynamics(CFD) method. The physical modeling and numerical calculation modeling are established for the simulation analysis and optimization of the air distribution and thermal comfort in the early design stage. The original scheme and the optimization scheme of the displacement ventilation system for a ship academic hall are compared and analyzed. The results indicate that the displacement ventilation system is characterized of the stratified flows and the vertical temperature gradient. Compared with the original scheme, the optimization scheme optimizes the cabin temperature, wind speed and thermal comfort index, effectively reduces the blowing sense, ensures the air quality around the human body, and its thermal comfort index also meets the standard requirements. The conclusions will provide reference in the application research of the displacement ventilation systems for ships.