Biodegradable Mg-xB i(x = 3, 6 and 9wt.%) alloys were fabricated by ingot casting, and the change of corrosion behavior of the alloys in the Hank’s solution was analyzed with respect to the microstructure using optical micrograph(OM), X-ray diffraction(XRD), scanning electron microscope(SEM) equipped with an energy dispersive X-ray spectrometer(EDS), electrochemical and immersion tests. The results show that the microstructures of the as-cast Mg-Bi alloys mainly consisted of dendritic α-Mg grains and Mg3Bi2 phase in common, with the secondary dendrite arm spacing(SDAS) decreasing significantly from 41.2 μm to 25.4 μm and the fraction of Mg3Bi2 increasing from 3.1% to 10.7%. Furthermore, the corrosion rate increasing from 1.32 mm·a-1 to 8.07 mm·a-1 as the Bi content was increased from 3wt.% to 9wt.%. The reduced corrosion resistance was mainly ascribed to the increasing fraction of the second phase particles, which bring positive effects on the development of pitting. Biodegradable Mg-xBi (x = 3, 6 and 9 wt.%) Alloys were fabricated by ingot casting, and the change of corrosion behavior of the alloys in the Hank’s solution was analyzed with respect to the microstructure using optical micrographs (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS), electrochemical and immersion tests. The results show that the microstructures of the as-cast Mg-Bi alloys mainly consisted of dendritic α-Mg grains and Mg3Bi2 phase in common, with the secondary dendrite arm spacing (SDAS) decreased significantly from 41.2 μm to 25.4 μm and the fraction of Mg3Bi2 increasing from 3.1% to 10.7%. Furthermore, the corrosion rate increasing from 1.32 mm · A-1 to 8.07 mm · a-1 as the Bi content was increased from 3 wt.% To 9 wt.%. The reduced corrosion resistance was mainly ascribed to the increasing fraction of the second phase particles, which bring positive effects on the development of pitting.