Abstract: Experiments on hydrogen directly mingled in the cylinder of a S195 diesel engine showed that along with the variation of hydrogen supply, the engine performance (chiefly the effective thermal efficiency and the specific fuel consumption ) was much better than that of the original diesel engine before using hydrogen, while the power produced was basically equal in both cases. In this paper, with the effective heat efficiency as the experimental target an optimized mathematical model was established on three factors, i. e the diesel fuel supply, the hydrogen supply and the fuel injection advance angle, at different engine speed and different compression ratio of engine. Optimum collocation scheme of the factors was also suggested.