Abstract: The control problem of unmanned aerial vehicle (UAV) suspended transportation systems (STSs) has become a popular topic in the field of both theoretical investigation and engineering applications. With the increasing complexities of the mission environment, it is difficult to accomplish a transportation task with a single UAV, and using multiple UAVs to collaboratively suspend and transport loads has become increasingly significant. Motivated by this, this work presents an event-triggered formation optimization-based control scheme for a multi-quadrotor STS, which can reduce the power consumption of the UAV formation by dynamically adjusting the UAV formation while achieving load trajectory tracking. In the developed scheme, the STS is decoupled into the suspended load and the multiple UAV subsystems based on the principle of timescale separation. It allows to design independently the virtual force command of the load and the actual control law, which are designed by using the sliding mode-like control method. Afterward, the formation optimization algorithm is presented. The obtained virtual force command of the suspended load would provide the receding horizon formation for the UAVs to minimize the sum of cable forces and the power consumption, where the constraints of cable tension, cable break, and collision avoidance are considered. Furthermore, the event-triggering mechanism is appointed for the formation optimization algorithm, and it reduces the solving frequency of the optimization algorithm and the computational burden of the system. Both the stability of the closed-loop system and the boundedness of the number of event triggers in finite time are confirmed by using the Lyapunov function method. Numerical simulations verify that the developed method can effectively reduce the energy consumption of multiple UAVs and reliably accomplish the trajectory tracking of the suspended load. Therefore, this work offers certain theoretical significance and practical application value for promoting the development of multiple UAV STSs. In addition, this work on the formation control of the STS expands the application scope of UAV suspension and enhances its transportation efficiency. In terms of cooperation suspension, how to generate and adjust the UAV formation to pass through narrow space still maintains profound research significance. Future work may consider the attitude loop of the suspended load, which plays an irreplaceable role in performing transportation tasks. Meanwhile, using a distributed communication protocol rather than a centralized one could distribute computational burdens to each UAV, which would better enhance engineering practicability.