Thermal runaway is a potential risk when using Li-ion batteries. A self-terminated oligomer with hyper-branched architecture (STOBA) has been demonstrated to be the most effective electrolyte safety additive for Li-ion batteries. Understanding the STOBA formation mechanism will contribute to improving such safety additives. Nuclear magnetic resonance (NMR) technology enables the molecular-level spectroscopic investigation of the STOBA synthesis reaction of N,N′-bismaleimide-4,4′-diphenylmethane (MDA-BMI) and barbituric acid (BTA) in solvent N,N-dimethylformamide (DMF) at 90 °C. In this study, the STOBA synthesis reaction was studied through a series of in situ NMR experiments at different elapsed times, and the experiments were repeated at different reactant ratios to confirm the reaction mechanisms. The quantitative NMR results revealed that the Michael addition reaction between MDA-BMI and BTA is the dominant mechanism. The Knoevenagel condensation reaction between BTA molecules and the thermal radical polymerization reaction among BMI molecules are minor mechanisms, and their contributions can be ignored. The products of the STOBA synthesis reaction were found to depend on factors such as the solvent, reactant type, concentration, reaction temperature, reaction time, stir rate, and order of addition.