Here, we conduct the experiments to investigate the hydrothermal decomposition of alanine more systematically than previous reports and study the effects of various reaction conditions including reaction time, reaction temperature, and raw material concentration on reaction routes and products in detail. Then, a quantitative kinetic analysis is used to evaluate the experimental results, in which the reaction order is calculated, and the total reaction rate constant is unified. The calculation of the total reaction rate constants for the disappearance, decarboxylation, deamination, and dimerization of alanine in this study proves to be correct and successful by re-analyzing and comparing with previous reports. Furthermore, a network model is constructed to gain more insight into the reactions during the hydrothermal decomposition of alanine. The results show that the reaction behavior of alanine hydrolysis under various hydrothermal conditions can be understood by this network model and the calculated rate constants. The method presented here to study the mechanism and kinetics of hydrolysis is expected to be applicable to other amino acids.