Abstract: Many scientists have already studied the focus of electron beam, and proposed several ways to analyse the electron beam, such as eikonal method, trajectories way, etc. Based on these,the paper uses the probability function to analyse the focus of electron beam, and treats the emission of electron as a random process. The focus has been attained from the probability of coordinates of trajectories.

Key words: focus of electron beam, eikonal method, trajectory method, probability function, random process