The Integrated Pressurized Water Reactor (IPWR) has great advantages in the development of small and medium reactors because of its compact structure and high inherent safety. The casing once-through steam generator (OTSG) has been widely adopted in IPWR. The heat transfer tubes are annular tubes, which adopt double-sided heat transfer, with strong hear transfer capacity. However, the heat transfer situation is complex as there are single phase regions, steam/liquid two-phase regions and superheated regions in the casing. It is therefore easy to generate two-phase flow instability during the start-up, shutdown or low power operation of the reactor. A lumped parameter analysis model for a casing OTSG of a land reactor is established by using the RELAP5 code, in order to investigate the influence of several parameter, such as system pressure, inlet subcooling and inlet throttling, on the flow instability. The calculation results show that the increase of the system pressure, the inlet subcooling and the inlet throttling under the certain conditions have a positive effect on the system stability. The study of the flow of the OTSG of the IPWR in this paper can also provide reference for the study of the OTSG for the marine IPWR.