基于小衍射源和衍射远场的特点，简化了瑞利索末菲标量衍射积分公式，并应用于分析非傍轴衍射光束的衍射远场总功率和光束传输因子特性。由于物理概念上的空间频谱的定义域局限在一定范围，基于此计算的非傍轴衍射光束的衍射远场总功率不满足能量守恒定律，基于此计算的非傍轴衍射光束的光束传输因子也与常识不符。当采用数学概念上的空间频谱分析上述特性时，两个符合常识的规律被重提，并以非傍轴基模高斯光束衍射远场总功率和光束传输因子的相对计算误差说明上述结论。

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.