Chemical conversion coating was prepared on AZ31 magnesium alloy profile in phosphate solution.Growth and morphology characteristics of coating were analyzed by SEM（scanning electron microscope）

XRD（X-ray diffraction）and mass-loss measuring method

and the formation mechanism of the conversion coating was approached.Mass-loss testing shows that there exists rapid corrosion dissolution of the alloy matrix accounting for more than 2/3 of the whole mass-loss in the early growth stage of the coating.Phosphate conversion coating exhibits two-layer structure

in which substrate layer consisting of interphase banding distribution of scaly and stripy structure exhibits orientation.The obvious slot between the bonding location of the two layer can be observed as a result of microstructure difference

which is responsible for the decrease of corrosion resistance of the substrate layer.Growth characteristics of phosphate conversion coatings were closely relevant to the microstructure features and chemical state of the matrix alloy.