In order to numerically simulate the diffraction propagation of convergence/divergence optical beams, the complex amplitude pulse response function of lens imaging is researched. A novel method called object-image conjugation translation is presented, in which the spherical wave phase factor in convergence/divergence beams is equivalent to a lens, and the spherical wave phase factor is removed in the diffraction propagation with the pulse response function and the object-image translation. This idea is simple but efficient, with a visual physical meaning. The simulation of the diffraction propagation of the optical beam with square aperture-lens assembly, proves the practicality and the validity of the idea.