The work aims to explore the effect of oxidation time on the structure and properties of micro-arc oxidation (MAO) coating on AZ31B magnesium alloy. The coatings were prepared in the silicate electrolyte system by constant pressure MAO method. The micro-morphology, surface porosity, thickness, roughness, friction properties and corrosion resistance of the coatings were studied by scanning electron microscopy (SEM), Image-J analysis method, thickness gauge, surface roughness tester, material surface performance comprehensive tester and salt spray test. With the prolongation of oxidation time, the porosity of the coatings decreased and reached to the minimum value of 5.404% after 25 min of oxidation. The thickness of the coatings increased with the prolongation of oxidation time, but the growth rate of the thickness decreased. The thickness of the coatings was 9 μm after 5 min oxidation and 10.4 μm after 25 min oxidation. The roughness and friction coefficient of the coating increased as the time increased. With the increase of oxidation time, the wear rate firstly increased and then decreased. The wear rate of samples obtained after 15 minutes of oxidation was the highest; the corrosion resistance of coatings obtained after oxidation for 5 minutes was the worst, and that of coatings obtained after 25 minutes of oxidation was the best. Under constant pressure, the prolongation of oxidation time can effectively reduce the surface porosity of the coating, increase the thickness of the coating, and significantly improve the wear resistance and corrosion resistance of the coating.