A series of transient analyses related to overpressure protection of an integral reactor is conducted to investigate the steam pressurizer (PZR) characteristic. The transient is considered the loss of load accident with delay reactor trip, and it yields thermally induced surge flow into PZR. The in-surged flow leads the excessive pressurization, and the system shall be protected from the overpressure by the Pressurizer Safety Valve (PSV). Mixing model and two phase depressurization model are considered to evaluate the PZR pressure behavior during the transient. Critical flow model is considered to be developed at throat of PSV when the saturated steam is discharged through the PSVs. Numerical analysis results show that the reactor can be provided overpressure protection with the PSV opening. Additionally, the flashing phenomenon is found during the depressurization, and the depressurization rate is reduced by the phase change effect after the onset of flashing.