In this study, modal parameter identification under multi-operational grades was proposed to
simultaneously eliminate the effect of environmental factors (traffic load, environmental temperature, environmental
humidity and wind load etc,) on the modal parameters of bridges. A cluster method was applied to
classify the monitoring modal parameters, obtained by using the long-term monitoring data of bridges, into
different grades. In each grade, the relationship between modal parameters and environmental factors could
be deemed as a simple linear relationship, and then the classified modal parameters could be applied to detect
the damage of bridges under the effect of environmental factors. Finally, the long-term monitoring modal parameters
of a practical bridge were applied to demonstrate the effectiveness of proposed method.