The load of the once through steam generator of the marine integrated reactor increases with the increase of the reactor power during the star-up. In the practical operation, it is specified that when the reactor power reaches a certain value, the steam superheat at the outlet of the steam generator meets the requirements, and the core power continues to be increased. And the secondary circuit side mainly depends on the increase of the feedwater flow to achieve follow-up. The operation scheme of the casing once through steam generator is determined based on the analysis of the flow instability for the marine casing once through steam generator during the start-up process. The corresponding modelling established by the RELAP5 code is then calculated and analyzed to verify the feasibility of the designed start-stop auxiliary system.