Deformation design is main content of shoe last CAD system. This paper introduces adaptive tangency (AT) algorithm, which is used to calculate the sectional curve when a known plane section shoe last under a discrete model, and describes its application in shoe last deformation design. Shoe last data can be converted each other in shoe last deformation design, which ensures data format standardized in whole CAD procedure. The detailed algorithm in deformation design is fulfilled and an actual example is given, which shows AT algorithm's application is successful.