Abstract: The development of the ocean has a great demand for the application of nuclear energy. The structure design,equipment and system layout of the small marine reactor are different from those of the land-based reactor. In order to adapt to the ship cabin and marine environment,this paper takes the main equipment structure and system of a modular small reactor as a reference,to obtain an improved design scheme and design parameters of the marine integrated small reactor. Using the software RELAP5,which is modified by adding the calculation module of the ocean condition,to introduce the ocean condition into one of the design schemes. And through deep cooperation with the ship designer,we can get the swing and incline conditions that the ship designer focuses on in the production design,the general design angle:22.5° and the ultimate design angle:45°. Under the condition that the control module is not added to the forced circulation and the steady state,the operation characteristics of the key thermal and hydraulic parameters are observed and analyzed according to the simulation results.