In order to evaluate the bearing capacity of composite airfield pavement reasonably, pavement classification number(PCN) calculation methods of composite airfield pavement in current specification are researched. The principle of equivalent displacement is established with the analysis of influence factors on airfield pavement bearing capacity. The formula of conversion thickness on composite airfield pavement is established with orthogonal experiment design and a large number of finite element calculations. Finally, the rules of various factors which influence the conversion thickness are discussed. The results show that composite airfield pavement can be simplified to an equivalent thickness single rigid panel when the PCN of composite airfield pavement is calculated. The concrete thickness on surface is the main factor influencing the results of PCN calculation. The difference in PCN calculated by different thickness conversion methods is significant. Thickness of asphalt concrete influnences the thickness conversion obviously. 