The air transportation network,one of the common multilayer complex systems,is composed of a collection of individual airlines,and each airline corresponds to a different layer.An important question is then how many airlines are really necessary to represent the optimal structure of a multilayer air transportation system.Here we take the Chinese air transportation network(CATN) as an example to explore the nature of multiplex systems through the procedure of network aggregation.Specifically,we propose a series of structural measures to characterize the CATN from the multilayered to the aggregated network level.We show how these measures evolve during the network aggregation process in which layers are gradually merged together and find that there is an evident structural transition that happened in the aggregated network with nine randomly chosen airlines merged,where the network features and construction cost of this network are almost equivalent to those of the present CATN with twenty-two airlines under this condition.These findings could shed some light on network structure optimization and management of the Chinese air transportation system. The air transportation network, one of the common multilayer complex systems, is composed of a collection of individual airlines, and each airline corresponds to a different layer. An important question is then how many airlines are really necessary to represent the optimal structure of an multilayer air transportation system.Here we take the Chinese air transportation network (CATN) as an example to explore the nature of multiplex systems through the procedure of network aggregation.Specifically, we propose a series of structural measures to characterize the CATN from the multilayered to the aggregated network level. We show how these measures evolve during the network aggregation process in which layers are gradually merged together and find that there is an evident structural transition that happened in the aggregated network with nine randomly chosen airlines merged, where the network features and construction cost of this network are almost equivalent to those of the present CATN with twen ty-two airlines under this condition. These could could shed some light on network structure optimization and management of the Chinese air transportation system.