Abstract: The thin-walled box of large tractor is easy to deform and requires high machining accuracy.During fixture design, improper selection of positioning datum and pressing point can easily lead to deformation of parts, which leads to the machining accuracy not meeting the drawing requirements.By using CAE software combined with machining requirements, multiple schemes of positioning and clamping points are compared, which vividly shows the influence of different schemes on the processing accuracy of the box.Through the optimization and improvement of the scheme and the actual machining verification, the machining accuracy required by the drawing is achieved, and the accuracy of fixture design and the machining accuracy of parts are improved.