To study the effect of fiber reinforced plastic (FRP) grid-engineered cementitious composite (ECC) matrix strengthened method on the deflection of reinforced concrete (RC) beams, flexural performance test was carried out on 10 RC beams. Each test variable which does effect on the deflection of RC beams strengthened with FRP grid- ECC matrix composite was analyzed, and a model for calculating the deflection of reinforced beams was derived. The test results show that the FRP grid-ECC matrix strengthened method can significantly improve the ultimate bearing capacity and flexural stiffness of the test beams, in which the ultimate load carrying capacity of the reinforced beam increases from 27.9% to 67.4%, the mid-span deflection decreases from 30.7% to 43.7%, and the reinforced beams occur suitable reinforcement damage with obvious ductile characteristics. The FRP grid has a strong influence on flexural performance of reinforced beams, and its grid thickness is proportional to the strengthening effection. The thickness, matching ratio and interface treatment method of ECC reinforcement layer have little effect on the flexural performance of the reinforced beam, and the sanding treatment improves the interface bonding performance of the reinforcement layer better than other interface treatment methods. The deflection calculation model of reinforced beams is derived based on the specification, and its calculating values agree well with the testing values, so the model is a reference for the deflection calculation of RC beams strengthened with FRP grid-ECC matrix composite.