Based on the perturbation theory, it was proved that the ratio of changes of frequencies is the function of structural damage location. Taking a uniform cross-sectioned simply supported beam for example, the ratio of changes of frequencies between the damaged and intact beam was used to locate and qualify beam damage. For different extents of damage, the changes of frequencies and ratio of changes of frequencies were studied and the effects resulted from the omission of second order items were discussed. Finally, the deficiency of this method was pointed out.