AbstractAmmonium magnesium phosphate monohydrate NH4MgPO4·H2O was prepared via solid state reaction at room temperature and characterized by XRD, FT‐IR and SEM. Thermochemical study was performed by an isoperibol solution calorimeter, non‐isothermal measurement was used in a multivariate non‐linear regression analysis to determine the kinetic reaction parameters. The results show that the molar enthalpy of reaction above is (28.795±0.182) kJ/mol (298.15 K), and the standard molar enthalpy of formation of the title complex is (−2185.43±13.80) kJ/mol (298.15 K). Kinetics analysis shows that the second decomposition of NH4MgPO4·H2O acts as a double‐step reaction:an nth‐order reaction (Fn) with n=4.28, E1=147.35 kJ/mol, A1=3.63×1013 s−1 is followed by a second‐order reaction (F2) with E2=212.71 kJ/mol, A2=1.82×1018 s−1.