As the Fresnel-Kirchhoff diffraction formula and the first and second solution of Rayleigh-Sommerfeld diffraction formulae coexist in the scalar diffraction integral formulae system, it is difficult to judge which one is the best. For the sake of comparing three diffraction formulae, the specialties of small diffracted source and far field diffraction are employed to simplify three diffraction formulae, the expressions of the scalar far field total powers in observation plane and vector far field total powers in observation hemisphere are presented. Based on the convergence of integrand, the applicable scopes of three diffraction formulae are introduced. And using the total power of diffracted source as standard, the computational precisions of the diffraction formulae are clarified. The analysis results indicate that, for the hypothesis of scalar diffraction field, three diffraction formulae are suitable for computing the paraxial scalar diffraction beam, but only the first solution of Rayleigh-Sommerfeld diffraction formula is appropriate for computing the non-paraxial diffraction beam. For the nature of vector diffraction field, three diffraction formulae are applicable for computing the paraxial and non-paraxial vector diffraction beam. Hereinto, for the normal incident plane wave diffracted by small circular aperture, the absolute value of relative calculated error of the far field total power which is computed by the second solution of Rayleigh-Sommerfeld diffraction formula is the least.