Abstract: To explore the blast loading acted on the underground protective arches, the Costantino model (MM) is introduced to consider the structure-medium interaction (SMI), in which the radius effect to the dynamic interaction is included. The displacement mode of the arch is assumed based on the equivalent single degree of freedom method, and the dynamic interaction equation of the structure is set up by the virtual work principle. The reflective coefficients are calculated by the ratio of the average peak stress of the dynamic loads to that of the free stresses. Influences of arch radius, structure stiffness, and medium modulus are discussed. Traditional reflective coefficient method is then modified by introducing new parameters to extend its applications to much more structure styles.