The Plasma Ion Mass Spectrometer (PIMS) is the first device able of measuring the absolute flux of ions in each charge state present in the edge plasma of tokamaks. Ion-optical simulations with the ion-trajectory code SIMION 7 are carried out to improve the characteristics of this in-situ measuring device. Critical properties like mass resolution and transmission of the PIMS are calculated and optimized for a range of plasma parameters, i.e. different ion temperatures Ti and sheath voltages Vs. Also the influence of small magnetic field misalignments is investigated.