Due to high strength-to-weight ratio and resistance to corrosion, glass fiber reinforced polymer (GFRP) composites have been used in various civil structures, where the GFRP profiles may be perforated to allow bolting, wiring, and pipelining, causing stress concentration in load-carrying scenarios. Based on experimental and finite element methods, this paper the stress concentration behavior of perforated and bolted GFRP square hollow sections (SHS) as a multi-layer orthogonal anisotropic material. In this study, GFRP SHS specimens with a circular central hole with a diameter of 6, 8 or 10 mm were subjected to uniaxial compressive loading, with or without high-strength steel bolts. The mechanical responses were monitored, and the stress concentration factors (SCF) were determined. The experimental results indicated that as the hole diameter increased from 6 mm to 10 mm, the SCF rose from 3.40 to 4.59. The presence of high-strength steel bolts exacerbated the stress concentration, with the SCF reaching a maximum of 6.07. Finally, a finite element model was developed and validated against the experimental results, and used to predict the mechanical performance of specimens with a broader range of hole diameters and bolt sizes, considering the effects of bolt preloads. The finite element results demonstrated that the SCF increased with higher preload levels, and appropriate preloading can reduce stress concentration compared to specimens without preload.