On the basis of the sophisticated phase-function approximation methods Delta-M and Delta-fit, the physical meaning of a combination approximation method Delta-combine is explained, combining the requirement of moment preserving of phase function and minimizing approximation errors within a specific scattering angle range. And the influence of the connecting point position for different requirements on the scattering intensity is analyzed. On this basis, taking Cloud and Haze scattering phase functions for examples, the radiation intensity and irradiance calculated by Delta-M, Delta-fit, and Delta-combine approximation methods are compared. The applicability of the three phase function approximation methods is statistically analyzed by these methodologies under different optical thicknesses. The results show that, as the connecting point moves backward, the root mean square (RMS) of absolute error of the intensity obtained by the Delta-combine method decreases, and the RMS of relative error of the intensity decreases first and then increases. The three approximation methods are all able to guarantee the energy balance. The RMSs of absolute error produced by Delta-M and Delta-combine are smaller than that of Delta-fit under different optical thicknesses. From the aspect of relative error, Delta-fit does well when the optical thickness is small, and the Delta-combine is best when the optical thickness increases.