The ratio of the power and the ratio of the power density are firstly defined to evaluate the quality of the beam beyond the diffraction limits. By use of the inverse transfer algorithm to calculate the phase distribution of an appropriate compensative phase, it is found that the reduction of the divergence of the laser beam beyond the diffraction limits and its power density of the diffraction central spot in the far field after reduction doesn't clearly decrease. This conclusion has more significances for practical applications of Gaussian beam beyond the diffraction limits in laser emitting optics. For the given intensity distribution in the far field with certain reduction ratio and limited energy loss, the reverse structure of phase plate could be calculated according to the inverse algorithms and the phase with continuous spatial distribution is quantified into step-shaped distribution so as to simplify the fabrication of a phase plate. This conclusion not only solves the contradiction of the reduction and the power loss, but also provides the theoretic basis for the experimental study and the practical application of the beam.