The surface morphology and corrosion resistance of the stannate conversion coating on AZ91D magnesium alloy at different grow stages were investigated by SEM（scanning electron microscope）,EDS（energy diffraction spectrum） and XRD（X-ray diffraction） as well as salt immersion testing.The results show that in the initial stage of chemical treatment,the MgSnO3 coating is deposited on the α phase very fast,resulting in the rapid increase of coating weight.After 10 min,the stannate conversion coating is deposited on the β phase with the decrease of growth rate.Then,the dissolution of conversion coating is accelerated on the α phase as a result of its loose structure,meanwhile,new coating is continuously deposited on the α phase,resulting in the dynamic balance.With increasing in chemical treatment time,the dissolution rate of coating is higher than deposition rate,which leads to the decrease of coating weight.The stannate conversion coating exhibits desirable corrosion resistance after chemical treatment for about 30 min.