Abstract: Based on the analysis of the advantages and disadvantages of the existing reactor types in the world and the application on ships, this paper suggests that the mature PWR can be used in the floating nuclear power plants on the sea, and the principle suggestions on the reactor power and refueling cycle are given. Taking the ship type floating nuclear power plant of single point mooring type as an example, the principle of compartment division is expounded, and the division is carried out according to the main functions of each cabin. The main restriction factors of the main scale are analyzed, and the general layout principle is expounded. The basic design principles of containment, safety enclosure, radioactive waste management system and biological shielding in the reactor compartment are emphatically introduced. At the same time, the design principles of some key systems such as secondary circuit, control room, power system and physical protection are introduced.