Abstract: To explore principal stress deflection effect on the stability of fault, based on the theory of internal stress field, the reason of principal stress deflection is explained, the mechanical structure model is established to calculate the internal and external stress fields, FLAC3D is used to calculate the deflection of stress principal axis when the working face is close to the fault from footwall, and to analyze the fault damage variable and its growth rate. Studies have shown that as the work continues to advance toward the fault, the principal stress induced by mining deflects twice in opposite directions in the internal and external stress fields; when the working face is close to the fault, the principal stress deflects to produce the shear stress parallel to the fault plane, which makes the risk of fault reactivation and instability higher. The theory of internal and external stress fields explains the deflection of the principal stress in the process of the working face advancing to the fault.