To increase the anti-oxidized properties of the micron-sized aluminum powder with shell-core structure, four kinds of micron-sized aluminum powder with different granularity(2.51, 5.20, 13.35, 24.02 μm)were slowly heated to 650 ℃ by a synchronous thermal analyzer at a heating rate of 20 ℃·min^-1 in the oxygen atmosphere to realize the enhancement of the alumina shell of micron-sized aluminum powder. The thermal response behavior of micron-sized aluminum powder before and after treating in slowly heating condition was investigated by the thermogravimetric analysis technique. The oxidation products at different stages were analyzed by scanning electron microscopy(SEM)and X-ray diffraction(XRD). Results show that after the the alumina shell is strengthened, the alumina shell of the micron-sized aluminum powder changes from amorphous state to γ state and the compactness increases. At the same time, the thickness of alumina shell of four kinds of micron-sized aluminum powder samples increases to 3.3-5.9 times of the original thickness. Under the condition of maintaining the high activity of micron-sized aluminum powder, the phase change and thickening of the alumina shell make the aluminum powder do not oxidize in the oxidative environment during slowly heating process below 1350 ℃, still maintain higher activity and increase the antioxidized properties greatly. The mechanism of slow oxidation of the micron-sized aluminum powder is changed from"shell broken"reaction mechanism with weight gain in stages to oxidation inhibition mechanism after the the alumina shell is strengthened.