A single-lens stereo vision system employing a biprism placed in front of the camera will generate unusual distortion in the captured image. Different from the typical image distortions due to lenses, this distortion is mainly induced by the thick biprism and appears to be incompatible with existing lens distortion models. A fully constrained and model-free distortion correction method is proposed. It employs all the projective invariants of a planar checkerboard template as the correction constraints, including straight lines, cross-ratio, and convergence at vanishing point, along with the distortion-free reference point as an additional constraint from the system. The extracted sample points are corrected by minimizing the total cost function formed by all these constraints. With both sets of distorted and corrected points, and the intermediate points interpolated by a local transformation, the correction maps are determined. Thereafter, all the subsequent images could be distortion corrected by the correction maps. This method performs well on the distorted image data captured by the system and shows improvements in accuracy on the camera calibration and depth recovery compared with other correction methods.