The synergistic inhibition effect of sodium silicate (SS) and sodium alginate (SA) on the corrosion of AZ91D magnesium alloy was studied by electrochemical impedance spectroscopy and potentiodynamic polarization tests in 3.5 wt% NaCl corrosive solution. Results indicate that the mixed SS and SA inhibitors have the best inhibition efficiency. Infrared and fluorescence spectroscopies were performed to examine the surface. It was found that SA was adsorbed on the surface of AZ91D magnesium alloy. Finally, the synergistic inhibition mechanism of SS and SA was proposed.