<p>This paper is devoted to developing a disturbance observer based fixed-time formation tracking control strategy for multiple quadrotor UAVs (QUAVs) under deferred time-varying constraint and external disturbance. Firstly, a shifting function is constructed to ensure the tracking error can rapidly enter the constraint range from the outside of constraint range. The constrained formation tracking error is further transformed into a new unconstrained variable by variable transformation. Secondly, by introducing a non-singular fast terminal sliding mode surface (NFTSMS), a fixed-time sliding mode disturbance observer (FTSMDO) is designed to tackle the external disturbance. Moreover, the fixed-time stability analysis of the FTSMDO and the closed-loop system are provided. Finally, compared simulations are carried out to illustrate the effectiveness of the proposed control scheme.</p>