Alpha functions of Soave-Redlich-Kwong (SRK) equation of state proposed by Soave, Two, and Luo were different in mathematic tendency. They were compared in modeling methane-alkanes equilibria with van der Waals mixing rule and Modified Huron-Vidal (MHV1) mixing rule, respectively. Results showed that Luo's alpha function was a little more accurate than Soave's, and Two's alpha function lacked accuracy in modeling methane-alkanes equilibrium. SRK equation of state was expanded as virial form, and then the equivalent terms were contrasted with terms of virial equation of state. Results showed that Soave's and Luo's alpha functions matched the tendency of virial coefficient better than Twu's, and Luo's alpha function matched better than Soave's in wide temperature range, which sustained the conclusions of phase equilibria calculation. Loo's alpha function keeps decreasing when T-r > 1 and becomes negative at sufficient high temperature, thus the conventional cubic equation of state expressed pressure as the sum of repulsion pressure p(R) (>0), and attraction pressure p(A) (<0) could be improved to be the sum of hard-sphere repulsion pressure p(H) (>0) and intermolecular force pressure p(I) (p(I) < 0 at low temperature and p(I) > 0 at sufficient high temperature).