The friction noise during braking of disc brake not only affects ride comfort, but also brings serious noise pollution. Friction noise features instantaneity and uncertainty, and so far there is no unified conclusion on the mechanism of brake friction noise. Based on the acoustic vibration test bench of disc brake, the law of variation of friction coefficient, friction fluctuation coefficient and equivalent A sound level under different braking pressures and at initial braking speeds were researched in this paper. Combined with the experimental analysis of micro-characteristics of friction disc, the occurrence law and mechanism of friction noise of disc brake were revealed from the tribological perspective. The results of the tests showed that the friction noise decreased with the increase of the initial braking speed, and declined after a period of rising when the braking pressure grew. The brake friction noise resulted from instability of the friction system with the friction pair having gullies and adhesion on surface which both had a positive correlation with the pressure and frequency of noise. The research results were of great theoretical significance and practical value for explaining the mechanism of friction noise of brakes and curbing the noise pollution of brakes.