Abstract: A damage identification method for multi-span beam structures is proposed in this paper. The conventional damage indicator methods such as flexibility matrix method have difficulty in dealing with the damage identification of complex continuum structure such as multi-span beams. The present method takes the change rates of certain diagonal elements of the structural flexibility matrix as identification indicator function. The "blanketing effect" of the identification indicator function with respect to the damage factors makes it easy to identify the damage of multi-span beam structures. Some useful formulas are derived and the sensitivity matrix of the indicator function is studied. A numerical simulation is performed and the identification results of several damage status are compared. The results show that the present indicator function method is very simple and effective. It is especially suitable for the damage identification of the multi-span beam structures such as bridges.