In this paper, we propose an effective data gathering protocol in an air-to-ground communication network employing priority-based contention window adjustment scheme. The proposed protocol adopts a conventional CSMA/CA. An unmanned aerial vehicle is used as a relay sink to collect data from a ground wireless sensor network. In our scheme, we divide the coverage area of UAV's beacon signal into many frames and each frame is assigned with different transmission priorities. In doing so, we introduce a priority-based optimized frame selection and circularly optimized frame selection scheme. In addition to POFS and COFS, PCWAS is proposed to define a lower contention window range to a higher transmission priority frame and a higher contention window range to a lower transmission priority frame. This approach not only minimizes the number of collisions; but also allows higher priority of data transmission to sensors that need to transmit first, so that the packet loss in communication link is dramatically reduced. Data transmission is handled from a higher to a lower priority frame. The simulation results show that the proposed approach achieves better system throughput, packet delivery ratio, and time delay.