The cooperation mechanism in unmanned aerial vehicle（UAV） ad-hoc network is studied. A cooperative time division multiple access （TDMA） protocol for UAV ad-hoc network based on dynamic relay selection is proposed. The protocol introduces the dual-queue cooperation mechanism when transmitting the relay packets， an independent media access control （MAC） layer relay packet buffer queue is introduced in addition to the network layer packet buffer queue. The protocol can also realize the dynamic selection of the default relay node and the helper relay node， so as to adapt to the heavy traffic load and the rapid change of network topology. The simulation results show that the through the dynamic selection mechanism of the relay node， the proposed cooperative TDMA protocol can obtain higher packet delivery rate and lower end-to-end delay than the traditional TDMA protocol and opportunistic cooperative relay time division multiple access （OCR-TDMA） protocol， when the network traffic load is heavy and the topology changes rapidly.