Usually the major airports have lots of air routes and scheduled flights. The intense competition turns passengers and cargoes of air transport to the major airports in a multi-airport system. It takes operating loss to many medium and small airports. From the view of airports, this paper analyzed the important factors of choosing destinations, and proposed a new indicator - air passenger traffic correlation. Finally, the optional destinations were collocated with the method of the Set Pair Analysis (SPA). Through the case analysis of Wuxi airport, the validity of method in this paper was verified. It provided the basis to choose destinations for medium and small airports.