According to the physical definition of Showalter Index (SI), four Calculation Schemes are proposed. Scheme A does not consider water vapor latent heat, resulting in large calculation errors. Although Scheme B considers the condensation latent heat, the curvature of wet adiabat is not considered in the computational method. Scheme B is superior to Scheme A, but the errors are still large. Scheme C not only considers the condensation latent heat, but also improves the computational method, considering the curvature of wet adiabat and the errors resulted from the derived temperature directly from pressure, with the result being superior to both Scheme A and B plan. Scheme D obtains the temperature of ascending air parcels through transformation by the pseudoadiabatic equation, and then figures out SI index. It is more precise, but its calculation is very complex, difficult to be used widely. Therefore Scheme C is preferred, can be put to operational use.