The Jominy end-quench curves of non-alloyed and alloyed steels can be predicted by nonlinear equation method. Without considering the interaction effects of alloying elements on the curves, however, the prediction for multi-alloyed steels may be different markedly from that experimentally determined. Hence, an improved mathematic model for predicting the Jominy end-quench curves of multi-alloyed steels was proposed by introducing a parameter named “alloying element interactions equivalent” to the nonlinear equations. With the improved model, the predicted Jominy curves agree very well with the experimental ones for more than 300 steel grades found in the literature. A computer-aided design system for hardenability of structural steels was then developed. The system not only can quantitatively evaluate the hardenability of structural steels, but also is very helpful to design steels and hardenability-aimed components. The optimal composition range of a new steel was so successfully designed as an example. Further research results show that the computer-aided design system so developed can also be used for monitoring of hardenability variation in steel industrial production.