In recent years, control barrier functions have become an important research branch in the field of unmanned aerial vehicle (UAV) safety due to their advantages in strong real-time performance, strong compatibility, strong constraint handling capability, and strong robustness. As UAV systems are mainly nonlinear control systems, ensuring the safety of UAV flights is crucial. This article first introduces the basic concepts of control Lyapunov functions, control barrier functions, and safety barrier certificates based on nonlinear control systems. Then, the application status of quadratic programming controllers based on control barrier functions in UAV systems is reviewed. Multi-UAV systems have higher task efficiency and adaptability than single UAV systems, so this article further reviews the application of centralized and decentralized safety barrier certificates in paired UAV systems. Finally, this article analyzes the technical challenges of applying control barrier functions in UAV systems and proposes future research directions that need to be further developed.