The charged particle beam tends to diverge and disperse when it propagates beyond the exit port of the accelerator owing to the mutually repulsive force exerted by the beam's similarly charged particles. The authors study the spreading of charged particle beams propagating in the vacuum of outer space in the effect of self-generated space charge force on the basis of the Lorentz Transformation of the Relativity Theory and Newtonian Mechanics, deduce a spread equation of propagation of charged particle beams, and present a method calculating the beam spreading radius. Then, the influences of the beam types, beam energy, beam current, initial radius and divergent angle on the beam propagation are discussed briefly. Finally, we point out that the electron beam with a high energy, weak current and big initial radius must be chosen as a weapon from a point of view of propagation technology. The results of the study are not only in agreement with open data now, but also provide a scientific basic for studying beam propagation technology .