The broken rock mass always causes instability and damage of tunneling face during shield construction. Therefore, a calculation method, based on strength reduction theory, modified ellipsoid theory, and limit equilibrium safety factor theory, is proposed for calculating the safety factor of shield tunneling face in broken rock mass using finite difference calculation method. The feasibility of the method is validated. The results show that: (1) The strength reduction theory is applicable for calculating the safety factor of shield tunneling face in broken rock mass. (2) The development of the broken rocks and support stress of tunneling face have a considerable impact on the safety factor.