The numerical simulation with FLUENT software is addressed for the natural convection heat transfer under the conditions of different airlayer thickness with 6 mm,9 mm,12 mm,14 mm and 16 mm. The heat transfer rate is compared with that by heat conduction. Gas types, Rayleigh number (Ra),the relative thickness of airlayer,δ/H,and the surface (ΔT) are main factors for natural convection heat transfer. The natural convection heat transfer could be approximately regarded as the pure heat conduction in the conditions of above airlayer thickness. The heat transfer coefficient of the insulating glass with simulation is approximate to that with empirical calculation.