Abstract: A method for determing the mechanical boundaries of a radial non-homo-geneous magnetic field analysing magnet is proposed, which has conical poles andcurved effective field entrance boundaries. The relationship between the mechan-ical and the effective magnetic field boundaries is obtained by using a series oftwo-diniensional magnetic field calculations to approximate the three-dimensionalcalculation of the field around the boundaries. On the basis of calculations, thereal mechanical boundary parameter of the magnet are determined, in which thecurved entrance boundary is replaced by a circular one. The probable deviationsand their effects on the optical parameter of the system are analysed in detailand an effective corrections means is proposed to make the parameter, especiallythe resolving power reaching the desired values.