FANET (Fly-Adhoc-Network) does not rely on pre-built infrastructure, and can form a temporary network through wireless links anytime and anywhere, which has been widely used in emergency communication and disaster relief. In order to solve the problem of signal transmission fading caused by heavy fog, heavy smoke and other harsh environments, and data packet transmission failure caused by high speed movement of unmanned aerial vehicle (UAV), a cooperative transmission mode is adopted to select a relay at the data link layer for packet forwarding, which effectively improves the network communication performance. Aiming at the problem of limited communication of UAV swarm in harsh environment, a cooperative medium access control protocol named energy consumption and channel gain cooperative medium access control protocol (EC-CMAC) for FANET is proposed. The protocol estimate the transmit power according to the channel propagation model, and further propose a relay selection strategy according to the estimate transmit power, the residual energy of the node, the direction and position of the node, and adaptively selecting the transmission mode. During cooperative transmission, it selected one-hop neighbors to forward the message, and extended the network lifetime by optimizing the energy consumption of the UAVs. The protocol is verified both statically and dynamically in MATLAB environment. The evaluation results of end-to-end delay, network throughput, network lifetime and packet transmission ratio testify that EC-CMAC can achieve longer network lifetime and higher packet delivery rate with little delay and throughput loss in harsh transmission environment and high speed movement of UAVs.