Structural damage detection methods based on modal strain energy usually require complete modes, however, only a small number of sensors are located on the structure in practice. In order to overcome this contradict, an improved structural damage detection algorithm based on modal strain energy is developed in this paper. By divided the contribution of higher modes into two parts, i.e., the static and dynamic contributions, the static term can be expressed by lower modes and dynamic term was approximately approached. It could be applied to detect structural damage location and quantity by using only first one or two modes. The three different simulated structures with the same damaged case are studied in present. The results illustrate that the present method is simple and effective with satisfying precision both in locating the place of damage and estimating the magnitude of damage