When discussing the free electromagnetic field, the author takes the electric field and magnetic field as two physical events on the time-space coordinate system. In line with the restricted theory of relativity, the author discusses the particle characteristics of the free electromagnetic field, and deems that the electromagnetic particle and the form of particle-captured energy are in perfect conformity with the Planck quantum assumption. In the ending part of the paper, the author has discussed the value of action exerted by dissimilar particles.