Abstract: The location and the shape of emitting surface in plasma ion source determine the extraction character of ion beam, and they always depend on the parameters of plasma, the configuration of electrodes and the extraction voltage, etc. They always automatically adjust to a certain state. In this paper, a 2D theoretical method to estimate these two factors, namely, an unmagnetized plasma can not penetrate a region with an applied vacuum dc electric field magnitude above a critical value. Thus the location and the shape of ion emitting surface can be found by solving the equation of Laplace of extraction system directly. Simultaneously, numerical simulation experiments of the location and the shape of ion emitting surface and the extraction beam current in different extraction systems based on