This paper describes a model approach for the simulation of a discharge electrode (DE) needle to charge particles using positive ions in an electrostatic precipitator. This includes the simulation of the electrostatic field, the space charge field of the ions and the flow field at the DE needle. The interactions of the fields, e.g. the reaction of the space charge on the electrostatic field or the electric wind are also considered in the model. To simplify and accelerate the simulation, a radial symmetry around the DE needle is partly assumed. The results of the simulation are validated by comparing the experimentally determined current-voltage characteristic with the simulated one, which show a satisfying correlation. Therefore, this model can be used as a basis for future particle flight simulation and further investigations.