To solve the problems of lacking comparative research on the improvement effects of different reinforcement methods when formulating reinforcement schemes for prestressed concrete small box girders and that existing bearing capacity improvement tests mostly focus on scale models with insufficient reliability of results, full-scale model tests and numerical simulation analysis of five 30 m span prestressed concrete small box girders with different joint reinforcement measures were conducted in this paper. Finite element models for two single reinforcement measures were established, and the improvement effects of different reinforcement methods on the bearing capacity of prestressed concrete small box girders were studied. The results indicate that compared with the unreinforced test girders, the damage ranges of all strengthened girders decrease, and their ultimate bearing capacities are improved to a certain extent. In terms of joint reinforcement measures, the joint reinforcement measure of pasting steel plates on webs, thickening bottom plates, and adding prestress has good effects in all aspects. The mid-span deflection decreases by about 42.7%; the structural stiffness increases by about 23%; the concrete stress decreases by about 28.6%; the cracking load increases by about 64.71%, and the ultimate bearing capacity increases by about 40%. Among the effects of single reinforcement measures, the measure of thickening the bottom plate (adding prestress) has good effects in reducing mid-span deflection, improving structural stiffness, reducing mid-span concrete stress, improving cracking load, inhibiting crack development, and improving ultimate bearing capacity. The test and simulation analysis results in this paper can provide a certain reference for the subsequent reinforcement reconstruction and bearing capacity improvement of similar bridges.