Abstract: Electron beam ion traps(EBITs) play an important role on extreme ultra-violet(EUV) and X-ray spectroscopy in laboratory astrophysics. Residual neutral gas in the EBIT trap center has a significant effect on ion breeding. In this work, we investigated the effect of residual neutral gas on the charge state distribution, as well as the diagnostic for the pressure in the trap center. The charge/energy exchange process between the neutral gas and highly charged ions not only affects the ionic fraction, but also plays an important role in fitting the excitation function (namely the relation curve of ionic fraction as a function of electron energies). By using the ionization equilibrium method on the measured excitation function, we have successfully estimated the residual neutral gas in the ion-electron interaction region of the trap center, as well as the vacuum pressure at the trap center.