Abstract: To investigate the biaxial flexural performance of concrete slabs stiffened with glass fiber reinforced polymer (GFRP) molded grids, this study carried out four-edge simply supported two-way flexural tests to explore the effects of single-point/two-point loading conditions and variations in GFRP grid height (13 mm, 25 mm, 30 mm) on the flexural behavior of concrete slabs. Meanwhile, numerical simulation of crack propagation was performed using ANSYS/LS-DYNA. The experimental results indicate that GFRP grids and concrete achieve excellent synergistic load-bearing performance, and their multi-crack development mode can effectively dissipate energy. Compared with plain concrete slabs, the ultimate bearing capacity and deformation capacity of the stiffened slabs are increased by more than 305% and 420%, respectively, with the specimens exhibiting brittle failure. When the grid height exceeds 25 mm, increasing the grid height yields a marginal improvement in the bearing capacity of the specimens, while the deformation performance decreases significantly. Under the two-point loading condition, the specimens are subjected to more uniform stress distribution, and the ultimate bearing capacity of the stiffened slab is 124% higher than that under single-point loading. Finite element analysis shows that the bearing capacity of the specimens increases with the grid height; however, when the grid height exceeds the section neutral axis, the growth rate slows down and the deformation capacity is significantly reduced. Therefore, to ensure the specimens possess favorable bearing and deformation capacities, a coordinated design between the GFRP grid height and the neutral axis position of the specimen section should be implemented when utilizing GFRP grids to stiffen concrete slabs.