A calculation method for an annual average loss of an external substructure reinforcement system based on interfacial shear stresses is provided. The calculation method includes: step 1: establishing a deterministic model for an external substructure reinforcement system, and generating seismic ground motion samples and structural samples by random sampling; establishing a probabilistic hazard model based on the seismic ground motion samples; generating a probabilistic vulnerability model based on interfacial shear stresses; establishing a total probabilistic loss expectation model by different seismic ground motion intensity levels; and using the total probabilistic loss expectation model to obtain an annual average probabilistic seismic loss of the external substructure reinforcement system based on the interfacial shear stresses. The calculation method can realize classified calculations of the seismic loss of the external substructure reinforcement system. Compared with the traditional seismic loss calculation method, the calculation method is more efficient in calculation.