Abstract: The small nuclear power plant was aimed as a reference in this work, and the models of the reactor coolant system, the intermediate and the secondary loop system were established by RELAP5/MOD3.2. The transition processes of single to double loop switch of the reactor and asymmetric loop operation were simulated to analyze the natural circulation characteristics. The results show that when the reactor switches from single to double loop operation and asymmetric loop operation under natural circulation, the core can rebuild stable natural circulation operation. In the case of double to single loop operation, core outlet temperature and flow rate reduce, but the flow rate is greater than 1/2 of the original flow rate. In the case of single to double loop operation, core outlet temperature and flow rate are consistent with those of steady state. In the case of asymmetric loop operation, the flow rates of core and fault loop reduce, but the flow rate of normal loop increases and the decrease in total flow rate is the difference between the flow rates of two loops.