In this study, the reaction process of the MAPC coating system is investigated. And the delayed curing mechanism of borax in the coating hydration system is proposed. The results suggest that the research process of magnesium oxide-monoammonium phosphate-water diluted ternary suspension system is applicable for the MAPC coating reaction process, which can be divided into three stages: NH₄H₂PO₄ dissolution, MgNH₄PO₄·6H₂O primary crystallization and secondary crystallization. Borax can delay the setting and hardending time of MAPC, reduce the pH value and lower the hydration temperature without forming any new hydration product. Borax can improve the fluidity of MAPC coatings, and borax content has an effect on the compressive strength of MAPC hardened paste. The retarding effect is controlled by the formation and ruptureof the protective film on the surface of MgO; the other is controlled by lowering the system temperature and adjusting the pH value of the coatings. In the hydration process, borax does not form new hydration products.