The invention discloses a deck type concrete arch bridge reinforcing method based on decrement of the mid-span deflection. According to the deck type concrete arch bridge reinforcing method based on decrement of the mid-span deflection, an inverted arch is arranged below an arch rib of a main arch ring of the deck type concrete arch bridge, and the inverted arch and the arch rib are connected through diagonal members; then the arch feet of the inverted arch and the arch rib are connected through anti-bending embedded parts and anti-shear anchor bolts, and prestressing force is stretched between a steering block on the lower edge of the inverted arch and an anchoring point of the original arch rib, so that an inverted arch structure used for reinforcing and the original arch rib form a rigid restrained supporting system; and the decrement degree of the mid-span deflection of the rigid restrained supporting system has a close relation with seven parameters of the to-be-reinforced arch and the inverted arch. By setting different variable values of the seven parameters, the ratio of the mid-span deflection the original arch to the mid-span deflection after reinforcement is used as the deflection change characterization, and a relational expression between the seven parameters and the deflection change characterization is obtained based on a finite element parameter analysis fitting method. By adoption of the deck type concrete arch bridge reinforcing method based on decrement of the mid-span deflection, the structural characteristic internal force value can be solved based on the relational expression, and thus the optimal scheme is selected for inverted-arch reinforcement.