The process parameters of a chromium-free conversion technique based on permanganate-phosphate solution on die cast magnesium alloy AZ91 have been optimized by orthogonal experiments. The effects of process parameters on thickness, corrosion resistance and morphology of the conversion film were discussed. The results show that the corrosion resistance of the conversion film can be improved by more than 8 times compared with that of the substrate when the concentration of ZnSO4 and NaF are 5 and 1g /L, respectively, and pH = 4. Furthermore, the pH value has significantly affected the thickness, morphology and corrosion resistance of the film. The film is loose and easy to fall off when the pH is too low. Meanwhile only the thin inner conversion film remains and the corrosion resistance is poor. When pH= 4, the film thickness reaches its maximum value, and the inner film adheres closely to the outer film, which significantly improve the corrosion resistance of the conversion coating. However, when the pH increases further, the film becomes dense, but the corrosion resistance increases slightly because of poor bonding between the inner film and the outer one