Abstract: The model of assembly tree was put forward to describe the assembly structure of mechanical products and their assembly relations.The assembly tree model can deal with the functional extension and strengthen the description of the product structure tree which was defined by the designer through assembly drawings.Product structure tree combined with the operations in assembly process,logic relations and other characteristics.The mathematical definition and modeling of assembly tree were proposed on the basis of its formalized description.The idea to take product function as the transforming carrier for the modeling of assembly tree,as well as the construction methodology and steps for assembly tree were also brought forward under the precondition that the mapping relationship between the product function and its structure was definite.The case in assembly process of a plunger injection pump was studied, and proved the assembly tree model feasible.