Abstract: Damage diagnosis method of building structure under operation is one of the hot issues in engineering currently. The relations of displacement modal, strain modal and curvature modal is expounded.With a concrete cantilever beam as the research object, the finite element model of cantilever beam is established by means of ANSYS. Through the simulation of damage cases under different damage location and damage degree, structural damage identification is studied.The aim of the work is to reveal the rule of strain modal in structural damage detection through the comparison between displacement modal modes and strain modal modes, and then to provide the theoretical basis for damage diagnostic study of large building structures under operation.