Objective The composite coatings were prepared on AZ91 magnesium alloy using the phosphate and sol-gel process.Methods Magnesium alloy was phosphatized, then coated with multiple SiO₂sol gel layers. Using the orthogonal experiment combined with electrochemical analysis method,we optimized the deposition process conditions of the sol gel film,and analyzed the surface microstructure and corrosion resistance of the composite coatings. Results The results showed that the optimized deposition process conditions of the sol gel film were as follows: volume ratio of sol formula ( TEOS : TEOH : H₂O : HCl ) 28 : 20 : 10 : 0. 35, gel temperature 30 ℃ , gel time 5 min, coating times 6. Under the optimal condition, the composite coating was smooth, with a few micro cracks and good binding force. Compared with the magnesium alloy substrate and phosphate coating, the corrosion current density of the composite coating was the lowest, and the electrochemical impedance was the largest. Conclusion The corrosion resistance of magnesium alloy was improved by phosphating sol-gel composite coating.