The 3D auto layout method is studied that is based on the numeric model for engine and it's accessories.The 2D LSI (Large Scale Integration) layout arithmetic is uesd for reference.The 3D minimum distance is changed into 2D using the method of "2D sector projection distance" according to the structure.This method simulates the real situation of the pipe connection of all the accessories well.It is developed that the software of 3D automatic layout is combined with the optimizing arithmetic and the rules of realities that have been summarized.So that the layout for 3D engine accessorise is realized.14 accessories have been placed as an example in this thesis.It takes nearly 20 minutes to complete it.