The work aims to improve corrosion resistance of magnesium alloy by studying an environment friendly surface treatment method. With yttrium nitrate as film forming material, a new rare earth salt conversion coating was successfully prepared on surface of AZ31B magnesium alloy in the method of chemical soaking. Surface morphology of the coating was observed with scanning electron microscope (SEM). Corrosion resistance of the film in 3.5% NaCl solution was studied by performing hydrogen evolution test and electrochemical test. The yttrium salt conversion coating was smooth and uniform and well covered. Uneven heterogeneous tumor like substances appeared on the yttrium salt film doped with graphene oxide. The film was intact and free from cracks. Results of hydrogen evolution experiments showed that the treated conversion film samples could greatly inhibit corrosion reaction. Polarization curve results showed that presence of yttrium salt conversion film enabled corrosion potential of magnesium alloy to shift positively by 150 mV; corrosion potential of yttrium salt film obtained doping with graphene oxide changed significantly, but corrosion current density was only 1/28 of the undoped film. Test results of electrochemical impedance spectroscopy also showed that graphene oxide-doped yttrium salt conversion film exhibited the maximum charge transfer resistance (Rct), 2485 Ω?cm2; followed by that of yttrium salt conversion film, 1224 Ω?cm2. Compared with the untreated magnesium alloy, charge transfer resistance of the two was improved significantly. Yttrium salt conversion coating can obviously improve corrosion resistance of AZ31B magnesium alloy, and addition of graphene oxide can further improve corrosion resistance of the conversion coating.