Abstract: Using the RELAP5-3D code, a stability model of supercritical water-cooled reactor (SCWR) was established. Based on this, two transients and two accidents were analyzed. The turbine bypass valves can hold the reactor pressure constant to make sure the reactor is safe. When loss of feedwater, the downward-flow water rods system can remove heat by heat conduction and supply the coolant to the fuel channel, they mitigate the core heat-up before the auxiliary feedwater system startup. So the downward-flow rods exhibit the safety of SCWR. Because there is no coast-down time, the hottest cladding temperature is the highest when the reactor coolant pump seizure occurrs, so the coast-down time plays an important part in mitigating the cladding heat-up.