Surface treatment with phenols can be a new controllable anticorrosion strategy for modulating the degradation rate of magnesium implants. In our previous work, the gallic acid (GA) and the hexamethylenediamine (HD) were used to fabricate the GAHD film on the surface of magnesium and exhibited good corrosion resistance. In this work, the mechanism of anticorrosion and controllability of the GAHD film was deeply investigated. The results of SEM, EDX and XPS show that large number of magnesium was chelated into the GAHD film by GA. And the higher content the GA was, the more magnesium was chelated. Furthermore, the results of electrochemical measures and immersion tests show that the GAHD film with higher GA content possessed stronger corrosion resistance. According to these results above, we found the GAHD film deposited on the magnesium was a kind of chemical conversion coating. In the body fluid environment, massive chelated magnesium would transform into insoluble magnesium hydroxide, which are known as a natural corrosion protection substance of magnesium and perform the effect of corrosion protection. And it could easily control the corrosion resistance of the GAHD conversion coating through regulating the content of GA in the coating.