Pyrolysis chemistry is important in both engine combustion and industrial utilization of various fuels. Understanding pyrolysis chemistry is challenging due to the large number of reactions involved and the explosion of intermediate species structures in the radical-driven process. Since the bond changes reflect the very core information on a reaction, automatic reaction classification based on reaction centers can be useful to peak at a simplified reaction view of a complex pyrolysis process. This work proposes and implements a scheme to build a reaction data set labeled with the reaction class for reactions from reactive molecular dynamics simulations using ReaxFF (ReaxFF MD) in generating global reactions in pyrolysis of realistic fuel mixtures. The major steps include the automated conversion of reactions into elementary-like reactions with a pseudosingle reaction center, automatic extraction of extended reaction centers, reaction class defining, and manual labeling. There are 46 reaction classes defined in total based on both pyrolysis reaction knowledge and reaction observations from ReaxFF MD simulations of realistic hydrocarbon fuel pyrolysis. With the effort to have as adequate number of reactions as possible labeled for each reaction class defined, 7862 reactions were manually labeled with reaction classes for the data set of 26,881 elementary-like reactions that cover major pyrolysis reaction classes of typical hydrocarbon fuel components of nparaffins, iso-paraffins, olefins, cycloparaffins, and aromatics. The reaction data set has been used in the scheme of SRG-Reax to build a semisupervised machine learning model of tri-training to predict the reaction classes of pyrolysis reactions. Through automated reaction classification, 30 major reaction classes involved in a total of 3479 pyrolysis reactions of real RP-3 fuel containing 45 components unravel the overall pyrolysis reaction characteristics of the fuel system. With additional reaction classes defined and reaction data labeled, the approach can be used for various fuels.