Abstract: Containment cooling capability verification test (COCOVET) is conducted to confirm air baffle removal code for passive nuclear power plant. The shrink ratio of passive containment cooling system (PCS) air passage between the test body and the prototype is 1∶4. In the COCOVET air convection heat transfer test, the flow around and separation of air at the tail end of the baffle are observed within the operating parameters of the PCS system, and the test working condition matrix is determined within the parameter range to study the mixed convective heat transfer characteristics of air and the containment wall. COCOVET air cooling test conditions were selected in this paper, and particle image velocimetry (PIV) was used to visualize the air flow in the containment annulus. By comparing the test data and code simulation results, the mixed convection heat transfer phenomenon of forced and natural air convection coexisting in the containment annulus was studied, the applicability of the safety analysis code of the containment was verified, the response of the containment after the accident was simulated, the cooling capacity of the containment was evaluated, and the feasibility of the optimization scheme of the baffle was demonstrated. The research results show that after the air reaches the tail area of the air baffle, most of the air in the descending section of the containment annulus enters the rising section 180° and flows upward along the baffle, while a small amount of air continues to flow downward along the cooling surface. In the area without baffle, the heat transfer mode of heating surface is mainly natural convection type, and the heat flux of heating surface is basically unchanged along the height direction. In the area with baffle, there is an obvious inlet effect in the rising section of the containment annulus, and the heat flux on the heating surface increases rapidly and then decreases. By using the lumped parameter and three-dimensional parameter modeling method and the mixed convective heat transfer empirical equation, the calculated values of the safety analysis code of the containment are in good agreement with the experimental values, which is suitable for simulating the mixed air convective heat transfer process. The containment pressure response analysis of the optimal scheme of the air baffle shows that the peak pressure of the containment is less than the design pressure of the containment and has a certain margin, which meets the requirements of the safety analysis of the containment. The research results verify the feasibility of the optimization scheme of the baffle in passive nuclear power plants, and enrich the research content of air mixed convection heat transfer.