Abstract: The important thermal phenomena involved in containment depressurization venting process of large passive advanced pressurized water reactor (PWR) containment vessel were studied by systematic key phenomena identification and ranking method. The phenomena identification and ranking table (PIRT) of containment depressurization venting process was developed. The results show that critical and frictional flow, twophase pressure drop, geometric dimension and flow state are the phenomena which have higher influence on the depressurization venting process of containment vessel in vent line and sparger. The condensation, heat transfer, geometric dimensions, fluid mixing, non-condensable gases and thermal stratification are the phenomena with high influence on the depressurization venting process in spent fuel pools. Based on the results of key phenomena identification and importance analysis and the experience and research results of the existing scale test bench, the similarity criterion which should be followed in the model test equipment was obtained. The similarity criterion provides design basis and theoretical basis for the construction of the verification test device.