Abstract: In order to simulate the focusing performance of the electron impact ion source （EI source） of the small quadrupole mass spectrometer developed by Lanzhou Institute of Physics, the effects of the focusing electrode parameters of the EI source, the distance between the plates, and the pore size of the plate on the ion focusing performance were studied by using the simulation software Simion8.0. The results show that the greater the voltage difference between the focusing pole voltage and the ionization chamber voltage, the better the focusing performance, and the focusing performance will deteriorate rapidly when the focusing pole voltage increases and gradually approaches the ionization chamber voltage. When the, spacing of each plate is increased at the same time, the ion focusing performance first gets better and then worse, the optimal focusing performance occurs when the distance between the electrode plate and the focusing electrode, as well as between the focusing electrode and the extraction electrode, in the ionization chamber is 1.25 mm. When the aperture of the lower plate and the aperture of the leading plate are reduced at the same time, the focusing performance will be significantly improved, and when the aperture of the lower plate and the leading plate of the ionization chamber remain unchanged, the aperture of the focusing plate will be changed, and the focusing performance will generally improve with the decrease of the aperture of the focusing plate, but the transmission efficiency will be reduced, and the best focusing performance mainly occurs when the aperture of the focusing pole is 1.6 mm or 2.0 mm. The above simulation results provide a data reference for the subsequent production of EI sources.