The influence of various factors (e.g., temperature, stirring speed, pH, and alginic acid concentration) on the crystallization kinetics of magnesium ammonium phosphate hexahydrate (MAP) was evaluated. The chemical potential gradient models were used to analyze the MAP crystallization mechanism and predict MAP crystallization kinetics. It was found that the temperature and stirring speed mainly influenced the kinetic rate constant, while alginic acid additives mainly influenced the thermodynamic driving force of MAP. The pH mainly influenced both the rate constant and the thermodynamic driving force, which is the key influencing factor of the crystallization rate and performance of MAP. The theoretical modeling indicated that the MAP crystallization mechanism was dominated by the adhesive-type growth mechanism. Furthermore, the crystallization kinetics of MAP was successfully predicted as a function of alginic acid concentration with high accuracy. This work is expected to provide important information for selecting appropriate operating parameters to control the performance of MAP in practical applications.