Recently the sextupoles have drawn much attention in literatures, for sextupoles can be used to compensate for the third-order spherical aberration of round lenses. In the present paper, the aberration theory of a combined magnetic round lens and sextupoles system with superimposed fields has been studied. Defining an inner product in the complex number field and using the complex representation and differential operators, we have treated both the Guassian dioptrics and the electron optical aberrations up to the fourth-order in this combined system comprehensively. All aberration coefficients have been derived in explicit form.