Abstract: The gaseous distribution in the ion source of gas chromatography-mass spectrometry(GC/MS) was studied with computational fluid dynamics(CFD) method using injection capillaries of different diameters and placed at different depth. This paper discussed the effects of the injection capillary’s depth and diameter on gaseous distribution in ion source. It is shown that the gaseous distribution in the ionization region of the electron impact ion source(EI) will be the highest when the injection capillary is placed at a special depth, and the diameters of the capillary has almost no impact on the gaseous distribution. The analysis of the gaseous distribution in the ion source provides an analysis method for the impact of gas density distribution on the ionization efficiency.