Abstract: Based on foreign and domestic research, a“power exponent”based classification and evaluation system for defects of existing railway tunnels is presented so as to analyze tunnel defects and evaluate the health of existing railway tunnels, as well as to provide reasonable and economical renovation plans. With different power exponents,the system gives the values of influence (values of damage) on the tunnel structure by a single defect under different development stages, and obtains a classification for the single defect. In addition, a comprehensive evaluation of multiple factors is available by means of a power exponent calculation, which provides a basis for the tunnel defect classification and health status evaluation. Finally, it is verified that this is a scientific method by practical cases.