Abstract: Using subchannel code PRTHA, some thermal-hydraulic parameters of IHNI-1 reactor were calculated. The fuel element temperature field and the coolant temperature distribution of the core were analyzed, and the calculation results of the power in the beginning of subcooled boiling were presented. The transient characteristics of IHNI-1 reactor were analyzed by using RELAP5 code, and the variation of the transient parameters with reactivity was also calculated. The analysis of the papern indicates that IHNI-1 reactor has the characteristics of inherent safety.