Disc brake squeal has always been a great challenge to the automotive industry. Based on the pin-on-disc system, a series of frictional squeal bench tests are carried out, which show significant time-varying characteristics on occurrence, sound pressure and frequency of frictional squeal. To investigate the generation mechanism of time-varying characteristics of frictional squeal, a four-degree-of-freedom (4DOF) lumped parameter model considering the time-varying tangential contact stiffness, the normal contact stiffness and the friction coefficient is established in this paper. Through both the system stability analysis and the transient response analysis, the time-varying frictional squeal is predicted successfully, and the generation mechanism and the key impact factors are also investigated in depth. The simulation results show that the time-varying characteristics of frictional squeal are influenced by the frictional contact characteristics of the friction pair, including tangential contact stiffness, normal contact stiffness and friction coefficient. And the disc surface run-out (SRO) is the main cause of the time-varying frictional contact characteristics. The uncertainty of sound pressure of frictional squeal derives from the uncertainty of the surface roughness, which is caused by the frictional wear and makes the occurrence of frictional squeal much easier.