Unmanned aerial vehicle (UAV) and intelligent reflecting surface (IRS) are anticipated to be widely applied for improving the spectrum and energy efficiency in the forthcoming wireless communication systems. To take full advantages of IRS-assisted UAV system, both beamforming and UAV's trajectory should be optimally designed. In this paper, we consider a challenging scenario where there are one UAV and several IRSs. For the IRSs-assisted UAV system, we formulate the problem as maximizing the received power at the ground user by jointly optimizing active beamforming at the UAV, passive beamforming at the IRSs, and UAV's trajectory over a given flying time. An efficient framework is proposed so that the joint optimization problem can be decomposed into three subproblems, which can be iteratively optimized individually. In particular, a closed-form expression is derived for updating the phase shifts of the reflecting elements, which helps develop a low-complexity algorithm. Numerical results show that our scheme outperforms other benchmark schemes, which corroborates the feasibility and effectiveness of our proposed algorithm.

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