This paper introduces the concepts of metroplex and flexible flight. We focus on the routing part of a complete flexible flight operation. The authors establish a network data structure based on which the optimal routing algorithm is developed for the flexible flights which fly from their original airports to destination metroplexes. Moreover, the metroplex routing algorithm is performed under the dynamic en route traffic congestion constraint, which is rarely studied in literature. We also propose an integrated model which combines the routing and scheduling according to the practical feasible air traffic control procedure. Besides the optimal metroplex routing and the scheduling, we also brief several other functional blocks in the integrated model and illustrate the data flow inside the entire system. Finally we present the simulation results which verify the positive effect of the optimal metroplex routing algorithm and show that the total delay reduction by the integrated system is considerable.