In order to study maximum bearing capacity of stone arch bridge reinforced by composite main arch, a circular arch bridge on reinforced equality section is analyzed. Characteristics of secondary loading on reinforcement structure and the mechanical properties of the two different materials of new and old structures are considered. The impact of the key structural parameters (height of reinforcing layer, width of reinforcing layer, longitudinal reinforcement ratio of reinforcing layer, and stirrup ratio of reinforcing layer) on the bearing capacity of reinforced main arch ring is analyzed. The results show that when the height of reinforcing layer is 0.25～0.50 times higher than original arch ring, the increment value of bearing capacity is quite obvious; when width of reinforcing layer is in excess of 67% over width of original arch ring, bearing capacity gradually increases; while longitudinal reinforcement ratio of reinforcing layer exceeds 1.162%, the increment value of bearing capacity is quite small; The effect of increasing stirrup ratio of reinforcing layer on bearing capacity of reinforcement arch bridge is relative low. In addition, the results recommend some structural parameters for the design of stone arch bridges reinforced by composite main arch circle method of this kind.