In order to research the stiffness degradation mechanism of RC T-beam under the condition of different damage and get the influence law of damage degree on dynamic characteristics of single beam, the dilapidated static and dynamic test on four pieces of RC simple T-beams is explained in the paper. Under different degree of damage these parameters of four pieces of RC simple T-beams with two kinds of reinforcement ratio, such as crack, mid-span deflection, basic frequency, dynamic stiffness, static stiffness are measured. The frequency and stiffness variation of RC T-beam is obtained. The functional relationships of T-beam between ratio of static stiffness to dynamic stiffness and load ratio , the frequency ratio and load ratio were established. Furthermore the numerical simulation calculation on test results is made by the finite element analysis. According to the above research, the damage mechanism and frequency variation of RC T-beam under vertical load are summarized.