The influences of H3PO4, KMnO4 and MnCO3 on the formation process of chromium-free chemical conversion coating at magnesium alloy/conversion treatment solution interface by using synchrotron radiation were in situ studied. Furthermore, we analyzed the surface morphology of magnesium alloys through the sweep electron microscopy. The result shows that the high-energy synchrotron radiation X-ray can penetrate through the magnesium alloy/conversion treatment solution interface, the formation process of conversion coating can be observed in real time using in situ imaging technology. The potassium permanganate can improved the formation of conversion coating significantly, and the surface of the coating is smooth with less cracks. However, manganous carbonate can limitely improve the formation of the coating.