In order to solve the damage detection problem, a damage detection method based on strain energy and evidence theory is presented in this paper. First, an evidence theory method is proposed to identify structural damage locations. Then, structural modal strain energy is utilized to quantify structural damage extents. In general, structural strain energy dissipation should be equal to the change of modal strain energy, according to this theory, a new energy equation is deduced, and structural damage extent can be obtained through the solution of the equation. The simulation results show that the method can perfectly identify damage locations and extents. Therefore, the proposed method is effective for the structural damage identification.