Abstract: Based on the theory of wide electron beam focusing with curved axis, a method for determining the off-axis point spread distribution is studied using the trajectories with curved axis. The result shows that the new method of calculating off-axis electron optical transfer functinon can reduce the amount of computation and increase the speed of calculation , yet retaining the same degree of accuracy when compared with the traditional method . It will have practical meaning in computing imaging systems with a large photocathode surface.