The paper describes a work that aims to prepare a layer with homogenized and densified fine-grained structure on magnesium alloy by using high rotating friction stir processing for enhancing corrosion resistance. The phase compositions, grain morphologies, texture, precipitates distributions, and corrosion morphologies of the processed zone are processted in detail by using XRD, OM, EBSD, TEM, and SEM. In addition, the thermal cycling curves, potentiodynamic polarization curves and Nyquist spectra of the processed zone are ascertained by using the K-type thermocouples and a triple-electrode electrochemical workstation respectively. A sound processed zone is prepared by using the single-pass high rotating speed friction stir processing. The as-received AZ31B magnesium alloy and the processed zone are composed of equiaxed grains α-Mg and needle-like β-Al12Mg17 phases. The surface appearances of the processed zones are glossy and flat. Compared with the as-received AZ31B magnesium alloy, in addition to the obvious texture, the mean grain size in the processed zone is significantly refined, and the distributions of the β-Al12Mg17 phases were more uniform and dispersed. In addition to the weakening of the texture, the mean grain size and the number of the β-Al12Mg17 phases are gradually increased as the rotating speed increased due to the sufficient heat input. The peak temperature in the processed zone is increased with the increase of the rotating speed. High rotating friction stir processing obviously enhances the corrosion resistance of the AZ31 magnesium alloy due to the formation of fined grain size, and homogenized and diffused distribution of β-Al12Mg17 phases. The processed zone produced at the rotating speed of 3000 r/min exhibits outstanding corrosion resistance. The corrosion potential of the processed zone increased from –1.49 V to –1.28 V, and the corrosion current reduced from 2.08×10–4 A to 9.51×10–5 A compared to those of the as-received AZ31 magnesium alloy. High rotating speed friction stir processing is proven to be the optimal path to enhance the corrosion resistance of the AZ31B magnesium alloy by effectively changing the microstructure characteristic.