Abstract: In this paper, a sub-synchronous modal energy (SSME) approach is proposed for the sub-synchronous oscillation (SSO) analysis and source identification of power systems with doubly-fed induction generators(DFIGs). First, the SSME structure of a power system with DFIGs is constructed and analyzed via generalized Hamiltonian theory. The SSME direction of a DFIG subsystem or a transmission line subsystem is proved to be a sufficient and necessary criterion for identifying SSO sources. On this basis, an SSO source location method based on the SSME of the subsystem is proposed. By monitoring the SSME direction of the subsystem, whether the SSO source is within the subsystem can be determined. Finally, the performance of the proposed SSME-based location method is demonstrated and compared with the existing transient energy flow (TEF) method under both negative-damping and forced SSO scenarios. Time domain simulation results validate the effectiveness of the proposed SSO source location method and demonstrate its better reliability than the conventional TEF-based method. In addition, it is expected that the evaluation of participation degree of DFIGs in SSO can be realized based on the proposed SSME method.