Abstract: In order to find out the influence of heating oxidation on the activation change of the micron-sized Mg-Al alloy powder, the effect of particle size and Al/Mg ratio on the activity of the alloy at 71℃ was investigated. And the variation of oxide shell thickness was calculated at the same time. The particle size, morphology and composition of the atomized micron aluminum-magnesium alloy powder were tested by the laser particle size analyzer, the scanning electron microscope, the energy dispersive spectrometer and the X-ray transmission scanning. The gas volumetric method was used to measure the activity of the alloy powder after the thermal oxidation test. The results show that the activities of the atomized micron Al-Mg alloy powder decreased with the increase of particle size and decreased with the increase of Al-Mg-Al alloy ratio at 71℃. The activities of the Al-Mg alloy powder remained almost stable after 24 hours of oxidation.