The security of unmanned aerial vehicle (UAV) communication systems is an increasingly important issue in lots of specific usage scenarios. To solve the challenging problem, this paper proposed to combine the Intelligent Reflecting Surface (IRS) technique with UAV and designed an efficient alternating optimization method to realize secure computation offloading of the UAV. Specifically, this paper proposed an algorithm is proposed to maximize the average security rate at the physical layer by jointly optimizing the accelaration of UAV, the beamforming of IRS and the transmit power alternatively. To address the problem, this paper decompose it into three sub-problems and use the semidefinite relaxation (SDR) and Gaussian randomization approaches to obtain the optimal solution iteratively until it converges. Based on the simulation results, it is evident that the proposed algorithm significantly enhances wireless communication security when compared to two benchmark methods.