Two popular interpolation formulas of calculating InGaAsSb quaternary alloy band gap energy are discussed, and the calculation results from them are presented and compared. It is found, after the two formulas have been converted into equivalent formulas in the same forms, that in them there is taken into consideration only the influence of bowing parameter in the Γ valley. In this paper, the effect of the spin-orbit splitting on the valence band is considered, and a new method of calculating the InGaAsSb band gap is proposed by introducing the bowing parameter of spin-orbit splitting. The results show that the introduction of the bowing parameter of spin-orbit splitting can improve the accuracy of the calculation results compared with the above two methods. When the fraction of In is less than 0.72, the calculation obtained from our method is most accurate.