In order to realize the strong column-weak beam hierarchy in existing reinforced concrete (RC) frames, a novel seismic retrofit technique (i.e., beam opening technique or BO technique) was proposed. The BO technique involves the cutting of a web opening in the RC T-section beam to weaken its flexural capacity, in combination with the installation of a fibre-reinforced polymer (FRP) strengthening system around the opening to offset the loss in shear capacity caused by the opening. While extensive experimental studies have verified the effectiveness of the BO technique, this paper presents a finite element (FE) study on the structural behaviour of RC beams with an FRP-strengthened web opening, for a more comprehensive and in-depth understanding of the performance of such beams. Two constitutive models of concrete were investigated, namely, the concrete damaged plasticity (DP) model and the brittle cracking (BC) model. Through detailed comparisons, it was found that for specimens failing in a flexural mode, the DP model is more appropriate; while for specimens failing in a shear mode, the BC model with the secant modulus of concrete adopted provides better predictions. A simple method for the selection of a proper FE approach is finally proposed.