The invention relates to the field of magnesium alloy surface treatment, particularly to a magnesium alloy chromium-free conversion film homogenizing pretreatment method, which mainly comprises pre-treatment, homogenizing treatment and homogenizing post-treatment, wherein the pre-treatment can remove greases and contaminants from the surface of a substrate and forms a layer of a thin protection film on the alpha matrix phase of a magnesium alloy, the homogenizing treatment can preferentially dissolve the large-size second phase on the surface of the substrate, and the subsequent homogenizing post-treatment can remove salt crystal grains and oxide skins remaining on the surface and can provide a certain activation effect. According to the present invention, the sample treated through the method can form the uniform core on the surface of the substrate in the conversion film solution, the grain distribution is compact, the corrosion resistance of the whole film layer is improved, and theproblem that the non-uniform film forming on the surface of the magnesium alloy at the early stage due to the influence of the second phase is solved.