A simplysupported reinforced concrete beam bridge was chosen to investigate it s dissipative characterist ics. The accumulated damage of the beam was simulated by a static loading test, and the change of dissipative characteristics with loading damage was tracked by a shoc k l oading test. The mechanics of damping ratio′s change with crack damage was anal yze d, and a new index of dissipative characteristic (adjacent periods′ diss ipative ratio) was presented. The analysis results show that the new index is s ensitive to the severity of crack damage and has good noise immunity. So it can be used as a damage index for simplysupported reinforced concrete beams.