It is well known that the paraxial approximation is no longer valid for the beams with large divergence angle and/or small spot size comparable with the wavelength. Thus, a rigorous nonparaxial treatment becomes necessary. As yet, a variety of approaches, such as the perturbation method, power-series expansion, angular spectrum representation and integral solution of the wave equation etc. have been proposed to treat the beam propagation behavior beyond the paraxial regime. In this paper the validity of series expansion corrections to the paraxial approximation of non-paraxial beams is studied. Detailed numerical calculations for the integral expression and series expansion of the wave equation are performed and compared. It is shown that the series expansion provides a valid correction to the paraxial solution as w0>0.2251λ, with w0 and λ being the waist width and wavelength, respectively. However, the series expansion solution delivers unrealistic results and may become divergent if w0≤0.2251λ. The applicable range of the series expansion approach depends on the waist width, propagation distance and order of the series expansion. The results obtained in this paper are useful for studying the propagation of nonparaxial beams.