With the rapid development of Unmanned Aerial Vehicle (UAV) technology, one of the emerging fields is to utilize multi-UAV as a team under autonomous control in a complex environ-ment. Among the challenges in fully achieving autonomous control, Cooperative task assignment stands out as the key function. In this paper, we analyze the importance and difficulties of multi-UAV cooperative task assignment in characterizing scenarios and obtaining high-quality solutions. Furthermore, we present three promising directions for future research: Cooperative task assign-ment in a dynamic complex environment, in an unmanned-manned aircraft system and in a UAV swarm. Our goal is to provide a brief review of multi-UAV cooperative task assignment for readers to further explore.