Abstract: The minimum distance between intersecting lines which are produced by intersected braces and chord has a great influence on welding quality and reliability for offshore jacket. An AutoCad LISP based a program to give an approximate result is developed to calculate and display this distance in order to replace complicated mathematical analysis and unstable manual visual inspects. The result of this program meets the demands of fabrication accuracy requirement and the program is faster, steadier than other methods and also easy to use, which facilitate the 3D modeling and engineering a lot.