It is well known that the paraxial approximation is no longer valid for the beams with large divergence angle and small spot size comparable with the wavelength. Thus,a rigorous non-paraxial treatment becomes necessary. In this paper,based on the scalar Rayleigh-Sommerfeld diffraction formula,the non-paraxial approximation solution and its validity are studied. Detailed numerical calculations for the exact solution,non-paraxial approximation solution and Fresnel approximation solution of circular aperture diffraction are performed and compared;It is shown that the non-paraxial approximation solution is rigorous and valid for the diffraction of a small aperture.