Abstract: Since the structure of deflection system is complicated, FEM (Finite Element Method) may be suitable to calculate the system. However, the deflection field has no definite boundary, it must be taken as an open boundary problem. In view of some peculiarities of deflection system, an improved method FEM-BEM is proposed for analysis. This method is different from ordinary hybrid method, it not only improves the accuracy of deflection traces, but also decreases the requirement for computer speed and memory.

Key words: deflection system, FEM, BEM, electron trace