In this paper some studying results of the magnetic field distribution of an electromagneticalsextupole applying to a system for correction of spherical aberration in round electron lens are described. Measurement data of the magnetic induction in every coordinates in the sextupole and thier approximate formulas forideal and practical sextupole are given by combining experimental measurements and theoretical analyses. Themagnetic field distribution at the edge of the sextupole is discussed in more detail.