To narrow the gap between the theory and practice in UAV (unmanned aerial vehicle) formation control, a formation flight experiment system is developed, which includes an X-Plane based hardware-in-the-loop (HIL) simulation subsystem and a flight experiment subsystem. In the two subsystems, the same autopilot and ground control station are used. By the HIL simulation, defects and bugs emerging in the simulation can be found and modified before field experiment. Therefore, the time and cost required by flight experiments are reduced, and the time from theory to application can be short-ened. Besides, communications between the vehicles and between the vehicle and the ground station are well tackled. The vehicles can share information via inter-vehicle communication without intervention of ground stations. Moreover, multiple UAVs can be monitored and managed by one ground control station. By network-based software and hardware designs, the experiment subsystem can be used to validate formation control algorithms based on various communication topologies. A leader-follower formation approach is designed to demonstrate the effectiveness of the entire experiment system. The results show that the autopilot validated by the HIL simulation can be applied to the flight experiment subsystem directly. The only work required by the actual formation flight is to perform a little fine adjustment on some control parameters.