The widely used PSRK model was modified to improve its predictive accuracy for the vapor-liquid equilibria (VLE) of highly asymmetric systems by introducing a so-called asymmetry factor, and a generalized expression for the calculation of the asymmetry factor was proposed. The modified PSRK model keeps the good accuracy of the original PSRK model for symmetric and slightly asymmetric systems, while gives much better predictions for the VLE of highly asymmetric systems. The model was tested with various binary and ternary systems, the results show that it works well, which is more accurate than the recently proposed LCVM model and Li et al.'s [Fluid Phase Equilib. 143 (1998) 71] modified PSRK model for the prediction of the VLE of highly asymmetric systems. As the modified PSRK model requires only the original PSRK group interaction parameters, and the expression for the introduced asymmetry factor is general, the new model has good applicability, and is convenient to engineering use.