Biodegradable Mg alloys have good bioactivity and suitable mechanical properties, which is desirable for application in the clinical fields. However, the biocompatibility of Mg alloys during the degradation process has always been a concern. In this paper, the corrosion behavior of AZ91 alloys in a simulated cell culture environment was studied, especially the insoluble corrosion product during the degradation was characterized, and the biocompatibility of the insoluble corrosion products was evaluated. The results of immersion test showed that the corrosion rate of the AZ91 alloy was low under the condition of high CO2 and humidity, especially in phosphate-buffered saline. Moreover, the insoluble corrosion product was MgCO3·3H2O, which showed a needle-like, circular aggregated, and square-radial feature. Meanwhile, MgCO3·3H2O showed poor cell biocompatibility by increasing the pH and Mg2+ content of the culture solution, which may affect the biocompatibility of Mg alloys.