A perspective projection model for prism-based stereovision was presented in this paper. The prism was considered as a single optical lens. By analyzing each plane individually and then combining them together, an affine transformation matrix which can express the relationship between an object point and its image was derived. Next, the perspective projection model between object point and its image was established. The methods for model parameter calibration, distortion correction and 3D reconstruction were also provided. The proposed method was based on optical geometry and could be used in a multi-ocular prism. Experimental results are presented to demonstrate the reliability and accuracy of our method which could be used in morphological measurement with high precision.