The invention discloses a method for determining the ancient shear strength of shale. The method comprises: a step of establishing a shear strength calculating model, which takes cohesive force, internal friction angle, maximum principal stress and minimum principal stress as variables, by performing derivation, based on a relation of stress Mohr's circle and direct stress and differential stress; and a step of calculating the ancient shear strength by introducing a time function into variables of cohesive force, internal friction angle, maximum principal stress and minimum principal stress through the shear strength calculating model. The method allows the ancient shear strength of shale to be determined.