This paper developed a simplified analysis model that includes a reactor primary loop, a secondary loop, and a passive residual heat removal system. Simulations were conducted to evaluate the startup of the passive residual heat removal system under two different hot standby conditions. When all of the pipes are filled with low-temperature, low-pressure water in the hot standby condition, the flow rate takes around 100 s to stabilize after start. In another hot standby condition, the pipes connected to the main steam line are filled with nitrogen gas, and a venting operation is required before startup. However, after the venting is completed, there are no significant fluctuations in the flow rate. Although the system in both hot standby states can meet the long-term heat removal requirements, considering the advantages of simple maintenance of water-filled pipelines, a parameter sensitivity study was conducted for the hot standby state with water, to investigate the effect of isolation valve operation time, initial liquid column temperature, and loop resistance on the transient response of the system. Longer valve opening times, higher internal friction, and lower initial liquid column temperatures extend the time for flow stabilization and delay the establishment of a stable flow state.