Abstract: The cooling ability of containment passive cooling system was calculated for post LOCA long term steady phase. The critical parameters and inner annulus features between containment outer surface and air baffle were studied. With a containment inner surface temperature hypothesis value, each heat transfer mechanism related to containment passive cooling process was analyzed, while the vaporized water mass and the water temperature on containment outer surface without vaporization were given. Since containment outer surface transferring heat to inner annulus air bonded two equations, close iteration calculation loop was formed, and by further adjusting original temperature hypothesis value, the iterative calculation was repeated again. The results show that the containment passive cooling capability is equal to 6.99 MW, while the post accident released energy is 6 MW at the corresponding accident phase, namely, for this concrete situation, thus the containment passive cooling design is effective.