Aiming at the problem of tool deformation during machine preheating, a tool thermal deformation measurement scheme based on image processing was designed to obtain the relationship of tool axial thermal error with the preheating time. High speed camera was used to collect tool deformation images, and Canny algorithm was used to extract rough positioning of the edges of the images by using MATLAB software. Subpixel fitting edge detection method was used to precisely locate the tool edge contour, that was, the least square method was used to fit the tool tip curve, and then the tool thermal deformation was calculated, which laied foundation for the subsequent thermal error compensation of machine tools.