Abstract: The throttling slip base made of soluble magnesium alloy often fails due to its rapid degradation rate during downhole service. Therefore, the surface of soluble magnesium alloy was modified to solve the problem. In order to understand the corrosion resistance of the surface-modified soluble magnesium alloy, the microstructure of surface-modified magnesium alloy before and after corrosion was observed with scanning electron microscopy(SEM). The phase structure of surface-modified magnesium alloy before and after surface corrosion was analyzed by X-ray diffraction (XRD) and infrared spectrometer. The corrosion resistance of the alloy in simulated well fluids at pH 3.0, 7.2, and 9.0 was evaluated by electrochemical and immersion tests. Compared with magnesium alloy, the corrosion potential of surface-modified magnesium alloy was positively shifted by about 1.1 V; the corrosion current density was reduced by three orders of magnitude, and the corrosion rate was reduced by about 3 mm/a in the simulated well fluids at different pH values. Compared with the simulated well fluid at pH 7.2, the charge transfer resistance of the surface-modified magnesium alloy decreased from 9.13×10⁶ Ω·cm² to 1.91×10⁶ Ω·cm² in the simulated well fluid at pH 3.0. The results show that compared with magnesium alloy, the corrosion resistance of surface-modified magnesium alloy in simulated well fluids at pH 3.0, 7.2, and 9.0 is significantly improved, while the corrosion resistance of surface-modified magnesium alloy in acidic simulated well fluids is greatly reduced and easy to degrade. The field test of the throttling slip base of the surface-modified magnesium alloy shows that the pressure performance and corrosion resistance of the slip base meet the requirements, and it is easy to be salvaged. The research and field test show that the corrosion resistance of the surface-modified magnesium alloy meets the requirements of soluble slip base, which provides a basis for its application in downhole throttling.