Abstract: Fine-interface mode synthesis method is a very effective Method for the dynamic analysis of large complex structures. However , the common subspace iterative method is inefficient when the reserved modes of a larger substructure is abstracted. In this paper the reserved modes of substructures are acquired with the help of WYD, and a method for the synthesis of the fine-interface mode of substructures is deduced.The paper then identifies the eignvalues of a given vehicle, and compares them with test results. The results show that the fine-interface mode method of synthesis based on WYD not only has a higher efficiency, but also a reliable precision.