This paper proposes a novel and promising MAC protocol applied for the new wireless sensor network based data collection applications that coordinate with an unmanned aerial vehicle (WSN-UAV). In this system, before communicating with the UAV by the CDMA-based transmission technique, the active sensors are divided into a number of sub groups with an appropriate priority each. We named the protocol as a Prioritized Frame Selection based CDMA MAC protocol (PFSC-MAC). The PFSC-MAC protocol incorporates both of the beacon signal receiving power and the variation trend of this factor. The simulation results have demonstrated that PFSC-MAC provides a low Packet-Error-Rate (PER) compared to the other schemes. In addition, the optimal number of priority groups has also been derived in order to obtain the minimal PER at each pair of the number of sensor in the network and the altitude of the UAV.