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This paper investigates an intelligent reflecting surface (IRS) assisted simultaneous wireless information and power transfer (SWIPT) system. Multiple IRSs deployed on unmanned aerial vehicles (UAVs) and ground building are considered in the proposed system for enhancing transmission of information and energy simultaneously. The optimization problem is formulated to maximize the average achievable rate over N time slots by jointly optimizing power splitting (PS) ratio, transmit beamforming, phase shifts and trajectories of UAVs. To tackle the non-convex optimization problem which can be decomposed into different subproblems, block successive upper bound maximization/minimization (BSUM) method is proposed for optimizing subproblems alternately. Considering the difficulty brought by LoS components in optimizing UAVs’ trajectories, double iteration algorithm is proposed which BSUM with fixed LoS components is applied in the inner layer and LoS components are updated in the outer layer. Meanwhile, feasible initial trajectories of UAVs are also analyzed for improving the efficiency in optimizing UAVs’ trajectories by using successive convex approximation (SCA). In order to provide theoretical foundation for further study, perfect channel state information (CSI) is considered firstly in solving the non-convex optimization problem. Furthermore, considering the limited ability in acquiring CSI at both IRSs and UAVs, optimization problem under statistical CSI is reformulated. Accordingly, with the aid of Jensen's inequality, double iteration algorithm can be also applied to tackle the optimization problem. Simulation results are provided to evaluate performances of proposed designs under different setups.