Abstract: The simplified system model of small module reactor (SMR) was built for core melt in-vessel retention (IVR) based on SCDAP/RELAP5 code. The transient heat transfer simulation for SMR during IVR process was performed, and the transient heat load of reactor vessel lower head was preliminary predicted and discussed in the present study. The study results show that the transient heat transfer characteristics during IVR implementation process can be divided into two stages including the dramatic heat transfer stage at the beginning of core melt slumping and the quasi steady state heat transfer after the crust formation. The IVR analysis for SMR shows that the heat flux density of lower head during the transient process can not exceed the critical heat flux density. The heat load when the steady state is achieved has significant safety margin. The study also shows that SCDAP/RELAP5 code has some deficiencies when used for IVR analysis.