This paper employs micron-level computed tomography to conduct CT scans of glass fiber reinforced plastic (GFRP) at four stages during in-situ tensile testing. The slices are reconstructed into three dimensions using a reconstruction algorithm. Different grayscale thresholds are applied to separate the material’s components, enabling the visualization of the three-dimensional (3D) pore structure. Finally, the 3D strain field of the material under various loads is analyzed using volumetric digital correlation (DVC). The results indicate that the number and proportion of pores increase linearly throughout the tensile process. The number and proportion of small pores rising first. Large pores only begin to expand under high load levels. Additionally, by correlating fracture images with the 3D strain field, it is found that high-strain regions coincide with the fracture locations of the specimens, which can effectively predict the failure locations of GFRP. The research findings contribute to a better understanding of the failure mechanisms of GFRP and provide theoretical support for its future engineering applications.