Magnesium phosphate coating belongs to a new type of high temperature resistance inorganic fireproof material, and also an excellent inorganic anti-corrosion material for industrial steel structures. This article focused on the mechanical properties of magnesium phosphate coatings after high temperature action. Through the experiments, the changes in mechanical properties such as hardness and bonding strength of magnesium phosphate coatings after high temperature (100 ℃, 200 ℃, 300 ℃, 400 ℃, 500 ℃, 700 ℃, 900 ℃) action, as well as the microscopic mechanism of the changes in mechanical properties were systematically studied. The results show that the magnesium phosphate coating has good integrity after high temperature action, and there are no defects such as powdering, blistering, peeling and cracking. Compared with room temperature, the mechanical properties of magnesium phosphate coatings after high temperature action decrease slightly. The magnesium phosphate coatings treated at 300 ℃ have the lowest bonding strength and the most obvious decrease in hardness. With the temperature increasing above 300 ℃, the mechanical properties of the coatings improve in different degree. Based on the microscopic characterization and thermogravimetric analysis at different temperatures, the four-stage high temperature evolution mechanism that caused the changes of the high temperature mechanical properties of the magnesium phosphate coatings was suggested.