Geometrically aligning two images is a basic image process in the machine vision field. In general, the geometric transformation model is selected according to the complexity of the geometric transformation between images, and the type of camera that takes the image is ignored. However, there are essential differences in the imaging mechanism between a line-scan camera and an area array camera, so the geometric transformation model of the area array image does not conform to the geometric transformation of the line-scan image. Therefore, according to the imaging model of the line-scan camera, we established the geometric transformation model of line-scan images. A line-scan image acquisition system was built, and planar objects were imaged by different line-scan camera poses. Then, taking the proposed model as the geometric transformation model, the line-scan images collected in this paper and the line-scan images of realistic application were respectively registered. As a contrast experiment, the homography of the area array image was adopted as the geometric transformation model to align these images again. Through the comparison of registration results, the correctness of the proposed geometric transformation model of line-scan images was verified, and the accuracy of the line-scan image registration was improved.