Aiming at the shortage of flexibility in dealing with short-term conflicts of UAVs, a UAV conflict prediction and collision avoidance system is designed. The system uses automatic dependent surveillance-broadcast system (ADS-B) to obtain the information of UAVs and their surrounding flight environment information, and detected the conflict risk of UAVs by predicting the flight routes of UAVs in the short term. A UAV collision avoidance method is proposed according to the conflict types that UAVs may send, the expected time of occurrence, and the surrounding flight environment information. The method is applied to solve the short-term complex conflict environment of UAVs, and the method based on rule tree and discrete action is used to calculate all the dangers in the airspace near the UAVs, so that all UAVs can avoid collision safely. Finally, the simulation calculation is used to simulate the short-term conflicts of seven UAVs in three-dimensional space, and the results verify that the method can complete the calculation of UAV collision avoidance successfully.