Abstract: In the integral design concept of small modular reactor, once-through steam generator (OTSG) was adopted. A lot of advantages were presented for OTSG, such as large heat transfer area, small equipment volume, high steam quality and so on. However, the overpressure risk of reactor coolant system (RCS) also increased because of the small water volume and the weak thermal inertia in secondary side when heat removal by the secondary system decreased. In addition, the compact configuration in modular reactor increased RCS overpressure risk as it reduced the thermal expansion capability of coolant. The overpressure risk of small modular reactor was studied by RELAP5 in this paper. The results indicate that overpressure occurs in RCS during the accidents of decrease in heat removal by the secondary system. Furthermore, turbine trip accident is the most dangerous overpressure transient. The RCS pressure is significantly related to the surge line flow area and reasonable design of surge line can mitigate RCS overpressure risk.