With the gradual opening of China"s low-altitude airspace, UAVs are increasingly used in various fields. Facing the rapid growth of flight flow, there is an urgent need to improve the safety of UAV operations, the key of which is to solve the problem of collision avoidance and early warning in UAV flights. In this paper, we establish a reasonable spatial model around the UAV, determines the conflict detection algorithm based on the longitude, latitude and altitude information of the UAV provided by the ADS-B message, screens the UAVs with conflicts in the horizontal and vertical directions, and selects the avoidance strategy of adjusting speed and heading. Through Matlab simulation experiments, the results show that the algorithm can effectively screen out conflicting UAVs and can perform three-level avoidance for them, which provides an effective guarantee for the flight safety of UAVs.