Carbon fiber-reinforced polymer (CFRP) is a widely used material for reinforced concrete (RC) beam strengthening. Because of exposure to severe environments and improper construction, CFRP sheets may separate from the bottom of RC beams. To analyze the influence of this type of interfacial defect on the mechanical properties of RC beams quantitatively and provide a reference for the rehabilitation of structures, this paper investigates the flexural properties of RC beams strengthened with partially bonded CFRP by experiments and analytical studies. To measure the degree of unbonded CFRP, a new parameter called the unbonded ratio was established, which is defined as the ratio of unbonded length to the total length of strengthening CFRP in the tension zone. Twenty-six RC beams were fabricated and tested in the present study, and the experimental variables were the unbonded ratio, thickness of the CFRP sheet, and anchorage method (vertical U-jacket, inclined U-jacket, and mechanical plate). The cracking load, ultimate load, load-midspan deflection curve, ductility, crack pattern, and failure modes of these specimens are discussed. Also, the coupling effect of the unbonded CFRP and anchorage method on the flexural performance of strengthened beams was investigated. Test results indicated that the ultimate load decreased with the increase of the unbonded ratio before the unbonded ratio reached its critical value. It was also found that the mechanical-plate anchorage and inclined U-jackets were superior to traditional vertical U-jackets in terms of load-carrying capacity and flexural stiffness and postponed the debonding of CFRP. Finally, a theoretical model for the ultimate load of RC beams strengthened with inclined U-jackets was proposed, which showed a good agreement with the test results.