Debonding failure is a common early damage form of aluminum alloy plate strengthened RC beam. In order to avoid the occurrence of debonding failure, the experimental research on the mechanism of aluminum alloy plate strengthened RC beam debonding failure was conducted. 24 RC beams were fabricated, and the aluminum alloy plates were adhered to the bottom of the RC beams with structural adhesive. In order to study the effect of additional anchoring on debonding failure, part of the test beams is equipped with chemical bolts or U-shaped hoops at specific positions on the aluminum alloy plate. Through the three-point symmetrical monotonic loading test of simply supported beams with aluminum alloy plates strengthened RC beams, four failure modes of RC beams strengthened by aluminum alloy plates are obtained:suitable reinforcement failure, over-reinforced failure, debonding failure at end plate or at mid-span induced by crack. The debonding failure is mainly caused by the excessive interfacial shear stress. Therefore, the test data of the strain gauge of the aluminum alloy plate were used to obtain the interfacial stress distribution curves, and the principle of interface shear stress distribution is analyzed. After reaching the maximum value at plate end, the stress quickly falls to around null. The interfacial shear stress in the cracking section of RC beam has a sudden change. The mechanism of debonding failure was analyzed:the mechanism of debonding failure at plate end is that after the interfacial shear stress reaches the bonding strength of the aluminum alloy plate and concrete, the interfacial shear stress will peel off the concrete in the protective layer; The mechanism of mid-span crack induced debonding failure is that the interfacial shear stress peels off the concrete fragment blocks from the beam body after the normal stress generated at the end of the concrete block is greater than the concrete tensile strength. On this basis, the discriminant formulae of two kinds of debonding failure were obtained, and the accuracy of the discriminants was verified by combining the test data. Above work provides a theoretical basis for the engineering application of RC beams strengthened with aluminum alloy plates.