Modal parameters are widely adopted to identify structural damages. This paper explores the mode jumping problems in damage identification for Beam string structure (BSS). The characteristics of structural mode are discussed firstly. It is found that the modal concentration is obvious for BSS, and this may cause modes after damage not in the same order as those corresponding ones before damage due to proper structural damages. The reasons for mode jumping are analyzed in detail. Based on the principle of relativity, the correspondence of the structural modes before damage and after damage can be determined by calculating the mode correlation coefficients. Numerical experiments show that this method is of higher precision, and can be applied successfully in structural damage identification for BSS.