Abstract: Flourier transform method is an effective method for solving Poisson equation in the numerical simulation of chemical release in the ionosphere, but the simulation accuracy needs to be improved. An improved method of Fourier transform to solve the Poisson equation is proposed. This method takes a Fourier transformation of the five point difference form of the Poisson equation and introduces an error correction term to make up the errors caused by the two order difference. The paper gives the detailed derivation process, and compares the improved method with the other methods. The result indicates that the mean square error of potential obtained by the improved method is 6 orders of magnitude less than the Birdsall methods and the potential derivative method, and the spatial distribution of the potential is closer to the analytical result. The proposed method meets the requirement of numerical simulation of chemical release in the ionosphere, and it provides a new method for solving Poisson equation in plasma particle simulation.