Abstract: The vacuum chambers of large-scale electromagnetic isotope separstors (Calu-trons) are of serious inward-concavity under the atmospheric pressure. The nume-rical simulation method is used to analyse the effect of inward-concavity on themagnetic field distribution and ion beam focussing characteristics. It is pointedout that strengthening the magnetic induction and adjusting the position of recei-ver adequately for a constructed apparatus can eliminate the effect of inward-concavity on the image widths of beams. This idea is verified through adjustmentof an electromagnetic isotope separator.