Abstract: ‍ ‍Intelligent Reflecting Surface （IRS） is a new green communication technology， improving the security of wireless communication networks by reconfiguring the wireless communication environment. In this work， we propose to minimize the transmit power of the Base Station （BS） for the IRS-assisted multiple-input single-output security communication system. Constrained by the secrecy rate of the legitimate user， we jointly optimize the transmit beamforming at the BS and the reflect beamforming of passive phase shifters at the IRS. A Lagrangian function and particle swarm nested optimization algorithm is proposed to solve the non-convex optimization problem with multi-variable coupling. We search the reflect beamforming at the IRS by particle swarm algorithm and solve the corresponding transmit beamforming by a Lagrangian function. In this way， we can obtain a suboptimal solution to the non-convex optimization problem. Simulations are performed on the required transmit power of the BS under different algorithms， showing that the proposed algorithm requires lower transmitting power.