In the design of electron beam deflection system with a large scanning field, determination of aberration is a practical problem. The relationship between the principal trajectory of electron beam exposure and aberrations has been investigated in combined round magnetic lenses and deflection system of SDS-3 electron beam machine. The double passageway principle in scanning system is used. Conditions of moving objective lens (MOL) are given and used for improving the performance of electron beam scanning system. The construction and the way to obtain the best trajectory have been analyzed. The structure of the electron beam focusing and deflection system of electron beam exposure machine is analysed with computer-aided design (CAD). The result of scanning system of this electron beam exposure machine indicates that the aberration of superimposed focusing-deflection system is so small that the dynamic correction is unnecessary. The electron trajectories are parameter of electrostatic deflection described in the vector form, the aberrations and structure equation are described in the integral form. The arrangement with 50 mm working distance at the 10 mm×10 mm deflection field with 0.005 rad aperture and 5×10-5 beam voltage ripple produces a total aberration disk of 0.03 μm before dynamic correction.