Abstract: In the practical application, space charge will be caused by the electron beam in the electron impact ion source, the influence of which should not be ignored. The passion distribution of electric field in ion source was achieved by the numerical method, and the movement of ions in it was also gotten. After that, with the use of the result from the calculation, the influence of the space charge on the performance of ion source was analyzed. Finally, an experiment was carried out to obtain the relationship between emission current of ion source and the intensity of different ions. The result of this experiment verifies the calculation and analysis mentioned above.