Abstract: A new method that combines Monte-Carlo method with the numerical integration method used for computing electron trajectories in vacuum to calculate the electron collection efficiency of the anode (or dynode) of an electron optical system is presented. This method can simulate the whole process of electron motion in a system. Taking the microchannel plate photomultiplier as an example, the collection efficiency is calculated, the influences of the distributions of electron emitting angles and energies are studied, and the limitation and extension of this method are discussed.