In order to obtain the critical rust expansion force of magnesium oxychloride cement reinforced concrete during cracking, the fracture behavior of magnesium oxychloride cement reinforced concrete under electric corrosion was studied based on fracture mechanics. According to Glinka Shen weight function method, the weight function of square plate with single crack at hole edge was obtained by using the known stress intensity factor at crack tip under reference load. Then the formula of stress intensity factor at crack tip of square plate with single crack at hole edge was deduced. Finally, according to the fracture criterion of concrete, the formulas for calculating the critical rust expansion force of concrete cracking under electric corrosion and its variation law were given. The results show that with the increase of interfacial crack length, the stress intensity factor at crack tip increases while the critical rust expansion force decreases. The stress intensity factor at crack tip decreases with the increase of the diameter of reinforcing bar.