Many advanced reactor designs incorporate the passive systems mainly to enhance their inherent safety. Some reactors even are designed to remove the accidental heat in the containment by an open natural circulation system, without using a mechanical pumps, e.g. HPR1000, APR+. The designs and safety analyses of these reactors are usually carried out by using the best estimate codes, such as the RELAP5, TRAC and CATHARE. These codes have been well validated by the pumped circulation systems and high-pressure natural circulation systems, but the applicabilities to simulate the thermal-hydraulic behaviors of open natural circulation have not been well proven by literatures.In the present study, one of the extensively used best estimate code RELAP5/MOD3.3 was used to simulate the thermal-hydraulic behaviors of an open natural circulation loop (PCCS-TF). The simulation results were validated against the corresponding experimental data performed on the PCCS-TF. The comparison results showed that RELAP5 code can successfully simulate most of the thermal-hydraulic transients, the flow mode distribution and the static characteristics of the open natural circulation system, in view of the simulation results reproduce in good agreements with the experimental data. Moreover, the influences of the structure parameters on the thermal- hydraulic behaviors were also researched, with the help of RELAP5 code to provide the technical supports for the design of the open natural circulation.