Abstract: A rigid-flexible coupled dynamic model is established to investigate the stick-slip flutter of a frictional brake system. Galerkin and numerical methods are then applied to reveal the influences of coupling relationships(i.e., internal resonant conditions) between the vibrations of block and disc on the dynamic behaviors of the whole system. The characteristics of the block flutter are also analyzed. The results show that above a critical speed, the vibrations of the block are close to the harmonic periodic motion due to the elastic vibration of the disc, and below a critical speed, stick-slip flutter is triggered. The strong movement coupling between elements of the system brings earlier occurrence of the frictional flutter and more intensive vibrations. Besides, resonances come out in a certain speed range due to the forced vibrations of block caused by the elastic vibrations of the disc.