Based on the specialties of small diffracted source and far field diffraction, the Rayleigh-Sommerfeld scalar diffraction integral formula is simplified to analyze the characteristics of the far field total power and the beam propagation factor of non-paraxial diffraction beam. Physically, as the spatial frequency spectrum of non-paraxial diffraction beam is confined to a finite spatial frequency, the calculated value of the far field total power fails to comply with the law of energy conservation. Besides, the calculated value of the beam propagation factor of non-paraxial diffraction beam is against the commonsense rule. Then, the spatial frequency spectrum in the mathematical concept is employed to analyze the above mentioned characteristics, and the two commonsense rules are put forward again. And it is clearly shown through the relative calculation errors of the far field total power and the beam propagation factor of non-paraxial TEM00 mode Gaussian beam.