This paper investigates the formation control problem of quadrotor unmanned aerial vehicles (UAVs) subject to completely unknown leader dynamics, parametric uncertainties and external disturbances in followers, as well as potential communication link faults a fault-tolerant formation control algorithm based on a distributed adaptive observer is proposed. Firstly, a UAV system model considering aerodynamic damping is established. Secondly, to address the problem that the system matrix of the virtual leader is unknown to the followers, an online parameter estimation method is designed. Building upon this, under the ”virtual leader-follower” communication topology and considering the constraint that some follower UAVs cannot directly access the virtual leader’s states, local interaction information from neighboring UAVs is introduced to construct a distributed virtual leader state observer. This enables accurate estimation of the leader state even when communication link faults occur. Then, an adaptive fault-tolerant formation controller is designed using the backstepping method to compensate for system uncertainties and guarantee formation tracking performance. Finally, simulation studies are conducted, and the results demonstrate the effectiveness and fault tolerance of the designed distributed observer and controller.