Abstract: The damage locating method based structural dynamics parameter has been a focus problem, but has no feasible solving method until now. The present paper finds that the variable for locating damage should be a type of local variable, and meet the following two basic requirements: 1) to be sensitive to local damage. 2) to be a monotonous function of location coordinates. The first order variational relationship about structural strain is found, from which a feasible mothod for locating damage can be obtained.