In order to solve the problem that the accuracies of dual-view three-dimensional (3D) video extensometer are different in two directions, and to avoid the problems of camera synchronization and high cost caused by using multiple cameras, a biaxial three-dimensional video extensometer based on single-camera four-view imaging is proposed. A rectangular pyramid prism is installed in front of a single camera, and the area to be measured can be observed from four views by a single camera. The 3D reconstruction of the measured points before and after deformation is carried out by using the strong constraint equation of multiple views, and the linear strain between the measured points is calculated. In order to avoid the influence of prism refraction on imaging, narrow-band monochromatic light is used for illumination. The measuring accuracy of the proposed biaxial video extensometer is verified by stainless steel tensile experiment. The experimental results show that the 3D biaxial video extensometer can accurately measure the linear strains in X and Y directions with high precision. The absolute error of strain is within 30 με after the average de-noising of linear strains in two directions. Comparing the measured mechanical parameters with the measured results of strain gauge, the relative error of elastic modulus and Poisson ratio is 0.56% and 1.8%, respectively.