Scaled particle theory was used to derive a general expression for the salt effect parameter, K, of isobaric vapor-liquid equilibrium for ethanol-water-1-1 type electrolytic systems, which appears in the Furter equation. This expression was essentially a sum of two terms: 1, the hard sphere interaction term calculated by Masterton-Lee＇s equation, 2, the soft sphere interaction term calculated by Y. Hu＇s molecular thermodynamical model, in which the diameters of nacked ions were replaced by that of solvated ions, the solvation coefficients (i.e., in the radio of the latter to the former) were taken to be adjustable parameters, their magnitude implies the ionic solvation rules. A correlation equation for the local dielectrical constant around central ions with liquid concentration was obtained by mapping out experimental points. The calculated salt effect parameters of 9 ethanol-water-1-1 type electrolytic systems were in good agreement with the literature values within the wide range of liquid concentration.