RELAP5/SCDAP/MOD3.4 is used to simulate and analyze the transient proceeding of in-hospital neutron irradiator mark 1(IHNI-1) reactor in accident condition.Large reactivity insertion accident and loss of coolent of pool accident are calculated and analyzed.The results show that IHNI-1 reactor is characterized with inherent safety. The negative coolant temperature feedback limits the nuclear power to an stable level and the reactor is safe under the accident.