In this work, a simplified kinetic model of the SO3 generation reaction is proposed with a test diesel engine, and the semikinetic model was further developed and applied to predict the acid dew point (ADP) temperature. In the model, the one-dimensional combustion model and the reaction kinetic model of O-radicals and SO3 generation were considered. The recommended kinetic constant equation was also provided. In addition, the evolution of O-radicals and SO3 and the effects of both on the semikinetic ADP model were also discussed. To the best of my knowledge, the introduction of dynamics into the ADP model is a new and noteworthy contribution. The research results indicated that the ADP model based on the semikinetic method improved the prediction accuracy of the thermodynamic ADP model. The key factors in the O2 dissociation reaction were the high-temperature environment and the presence of flame and ignition. The O-radical concentration played a leading role in the SO3 formation reaction, and the SO3/SO2 conversion ratio was less than 10% in the cylinder.