The web's local buckling is a major failure type of pultruded glass fiber-reinforced polymer (GFRP) bridge deck subjected to concentrated wheel loads. Three kinds of external actions-that is, in-plane shear force, local compressive force and bending moment-are coupled at web. Besides, the couple ratio of those three external actions is changed due to varied wheel's loading position, resulting in a more complicated web buckling mechanism. Hence, a study of the web buckling behavior was firstly conducted through two experiments with different concentrated loading cases, namely mid-span loading and quarter-span loading. After then, parametric analysis of FE model with detailed simulated web-flange junctions (WFJ) were performed to reveal a more comprehensive understanding. The experimental results showed that both specimens went through buckling failure of middle web, post-buckling strengthening owing to structural redundancy, and final failure caused by crack propagation on the top flange. The parametric analysis showed that the compression buckling and shear buckling coupled together, and that the compression buckling is dominant. In addition, as the shear-span ratio decreased, the principal compressive stress and the shear buckling became more pronounced, and consequently the GFRP bridge deck's bearing capacity reduced.