This paper proposes an event-triggered communication mechanism-based distributed fixed-time three- dimension (3-D) formation control scheme of underactuated autonomous underwater vehicles (AUVs) in the presence of uncertain dynamics and unknown time-varying ocean environmental disturbances. Firstly, a fixed-time disturbance observer (FxTDO) is designed to estimate the compound disturbances lumped by the uncertain dynamics and unknown time-varying ocean environmental disturbances and the estimation errors of the FxTDO will converge to zero within a fixed settling time. By constructing an event-triggered communication mechanism, the communication frequency between AUVs is reduced and the communication resources of the network are saved. Furthermore, based on the above FxTDO and event-triggered communication mechanism, a distributed event-triggered fixed-time 3-D formation control law is designed by synthesizing the dynamic surface control technique and fixed-time control theory. Both the existing fixed-time disturbance observers and control laws have two power terms, the designed FxTDO and fixed-time formation control law in this paper only retain the power terms with faster convergence rate to reduce the difficulty of parameter adjustment. Theoretical analysis indicates that the designed control law enables AUVs achieve the desired 3-D formation control within a fixed settling time. Simulation and comparison results verify the effectiveness and superiority of our control scheme.