Abstract: Based on the concept of secondary containment of onshore nuclear power plants, the concept of safety enclosure of floating nuclear power plants is introduced in this paper. The radioactive containment “containment + safety enclosure +reactor compartment” is proposed. This paper studies the evaluation of the design idea of bypass leakage for safety enclosure and proposes a method for identifying the bypass leakage path and determining the bypass leakage rate. The design basis of negative pressure for safety enclosure is given in the end, which can be referred in the design of the ventilation system for the floating nuclear reactors in later stage.