In the dual casing sidetracking,the oil casing wall,cement ring,technical casing wall and rock formations’ hardness are different,furthermore,in the process of dual casing sidetracking the bit in contact with the oil casing wall,cement ring,technical casing wall and rock formations are different,bit bending moment certainly exist. Considered the situation of that the sidetracking bit in actual contact with the oil casing wall,cement ring,technical casing wall and rock formations,established the bending moment mathematical model of the dual casing sidetracking drill bit,and combined with examples using MATLAB software solved the relationship between the drill bit bending moment and the drill bit horizontal moving distance under that Three different working conditions was given. Further reference the theory of metal cutting mechanics,researched cutting force and cutting torque of the drill bit in sidetracking the process,checked the sidetracking bit strength,provided technical advice to prevent the drill bit holded in and stuck fracture accident occurred in the dual casing sidetracking process. The analysis and calculation could provide the reference to designing the sidetracking bit and determinating construction parameters in the dual casing sidetracking process.