In wireless sensor network (WSN), in order to improve the energy efficiency of sensor nodes (SNs) and extend the service life of WSN, UAV is usually used. UAV as a data collector. However, due to the characteristics of high energy consumption in traditional UAV flight trajectory setting, in addition, some nodes have poor communication channels in geographical characteristics. To solve the above problems, a WSN energy-saving data acquisition method based on IRS assisted UAV trajectory optimization is proposed. The method uses IRS to enhance the UAV′s data acquisition capability through the signal sent by the reflection sensor. The energy efficiency of WSN nodes is improved by optimizing UAV trajectory. Specifically, the IRS-assisted UAV air-ground channel model is first constructed, and then the WSN energy consumption objective function is constructed for UAV flight trajectory and wake-up mechanism constraints. By using block coordinate descent technology, the objective function is transformed into an optimization problem for UAV flight trajectory and SNs wake-up scheduling strategy. Through simulation, compared with the three typical UAV trajectory optimization methods, the energy consumption generated by this scheme is reduced by 91.0%, 61.5% and 41.6%, respectively, which proves that the WSN energy efficiency is significantly improved by this method.