Based on the mechanism of the relative change rate of mode shape (RD), a new damage identification quantity called the vibration mode slope change rate (RD’) is proposed. The new quantity is analyzed with the fundamental principles of function differential method. Dynamic test is carried on 5 RC frame beams, which are subjected to an increasing static load in the middle to introduce cracks. After each load step, unloading, and an experimental dynamic monitoring is performed on the beam, then reloading to the next step. Modal analysis is carried out on frame beams under each load step, and modal displacements are given, the vibration mode slope change rate graphs of the RC beams are calculated, which can be used to detect multiple-damages. By means of SAP2000, the comparison analyses between RD and RD’ indicate that RD’ can effectively avoid the careless judges and misjudge in the damage identification of frame beams.