The end bearing capacity of a rectangular steel tube can be substantially increased through local strengthening by bonding carbon fiber reinforced polymer (CFRP) plates to the webs of the steel tube. Existing studies have revealed that failure of such a strengthened tube generally occurs by debonding of the CFRP plates from the steel tube and the effectiveness of this strengthening method depends significantly on the properties of the adhesive. This paper presents a finite element (FE) study on debonding failures in such CFRP-strengthened rectangular steel tubes subjected to an end bearing load. In the proposed FE model, a bond-slip model developed by the authors' group for CFRP-to-steel interfaces with linear adhesives together with a mixed-mode cohesive law are employed to predict debonding failures. The FE model is shown to closely predict the experimental behavior of such CFRP-strengthened tubes.