Abstract: The containment external heat environment of a large nuclear reactor passive containment cooling system (PCS) was studied. A natural convection heat transfer model of the containment narrow space was established, the flow field was simulated based on Navier-Stokes (N-S) equations, also the influence of the outlet height on passive containment cooling system cooling performance was studied. The results show that: when the inlet air is under 308.15 K, 1atm conditions, the area weighted average temperature of criterion type containment outlet is 330.33 K, the cooling air mass flow is 275.85 kg/s, the took away heat is 6160 kW; with the increase of outlet height, the mass flow and heat transfer of containment outlet are increasing, but the slope of the changing curve is decreasing, and finally leveled off, while the cooling air temperature efficiency which is a measure of cooling capacity decreases linearly, air resistance increases linearly, accordingly, sthere is an optimal design.