Steel stranded wire mesh and polymer mortar (SMPM) is a novel technique recently developed for structural strengthening. To investigate the feasibility of strengthening reinforced concrete (RC) bridge box-girders with SMPM, bending tests on three scaled RC box-girders were made, based on which formulas for predicting the bending and debonding capacities of the strengthened RC box-girders are proposed. First, the box section is represented by the T section using the effective width factor in the elastoplastic stage. Then, based on the equivalent height of compressive region, bending failure modes of the strengthened girders are classified, and equations regarding the ultimate bending capacity corresponding to each failure mode are established respectively. To account for the debonding failure observed in the tests, a shear-capacity-based model is proposed. Comparison was made between the analytical and existing test results.