Polyurethane prepolymers based on 1,5-naphthalene diisocyanate (NDI) have limited storage stability. To investigate the reasons for this, kinetic experiments and quantum chemical calculations were used to study the reactivity of NDI. The results show that the reactivity of NDI is not higher than that of commonly used aromatic isocyanates such as 4,4'-diphenylmethane diisocyanate (MDI) and 2,4-toluene diisocyanate (TDI), and even lower than that of aliphatic 1,6- hexamethylene diisocyanate (HDI). Combining experimental data and literature analysis, it is found that higher reaction temperature (120~130℃) in the synthesis of NDI-based prepolymers can lead to the branching side reaction of isocyanates with urethane group, resulting in gelation during the storage or casting of prepolymers. Stable and storage-resistant NDI- based polyurethane prepolymers can be obtained by adding the inhibitor phosphoric acid during the synthesis process, and the molecular weight and viscosity of the prepolymer as well as the mechanical properties of the polyurethane elastomer obtained after curing decrease with the increase of phosphoric acid content.