This paper proposes a flexible structure energy decoupling method based on generalized elastic force work， to obtain elastic vibration coupling frequency band diagram. The coupling degree of elastic vibration mode is evaluated for matching their elastic modal design. A double-stage vibration isolation system of powertrain is studied. Firstly， a multi-degree-of-freedom dynamics model of a double-stage vibration isolation system is established considering the elastic modes. Then the flexible decoupling energy decoupling method is used to quantify the vibration coupling degree between the power pack and the intermediate frame elastic modes. The coupling band width curves at different elastic frequencies are obtained. The results show that the anti-phase vibration between vibration modes can mutually reduce the modal equivalent mass， in-phase vibration modes can increase the modal equivalent mass， thus changing the national characteristics of the system because of the coupling essence between elastic modes. Based on the elastic vibration coupling band curve， the avoided coupling band width between the powertrain and intermediate frame elastic mode frequencies can be determined to follow the instructions of the vibration isolation system. The elastic mode matching method between the powertrain and intermediate frame can be accomplished. The vibration performance test demonstrates that the method can effectively control the elastic vibration coupling degree between the powertrain and the intermediate frame with good vibration isolation performance. Therefore， the method can provide a theoretical support for similar problems and reference for the elastic modal matching of similar models.