The ion flow field under the UHVDC power transmission line is one of the important evaluation indicators of the electromagnetic environment. Based on the Newton-Raphson method and the upwind FEM, this paper proposes a fast and stable algorithm for solving the ion flow field. First, FEM is used to solve the Poisson’s equation, and then the space electric field information is used in upwind FEM to calculate the current continuity equation. Subsequently, an iterative convergence algorithm based on Newton-Raphson is used to update the surface charge density of the conductor. Repeated iteration of the above three parts can achieve rapid and stable convergence of the iterative process. Meanwhile, the influence law of wind speeds on the distribution of the ground total electric field and ion current density under the ±800 kV transmission line is studied in this paper. The experiment results in this paper verify the effectiveness and correctness of the algorithm. The research results show that compared with the iterative convergence control factor method and the traditional method, this algorithm is not only insensitive to the initial value of the wire surface charge density, but also has a higher rapid convergence ability. The research conclusions of this paper can improve the stable convergence ability of ion flow field calculation, and realize the fast and accurate calculation of the electromagnetic environment of UHVDC power transmission lines.