In order to study the time-varying and uncertain properties of automotive brake squeals induced by friction, a drum brake squeal experiment is carried out on a brake noise and vibration test bench，and the squeal signal is analyzed with statistical methods. A data processing flow chart and a set of evaluation indexes of squeal uncertainty are established. Meanwhile, the impact of frictional conditions on time-variability and uncertainty of squeal is also investigated. It is found that the frequency and sound pressure level of squeal show great time-variability and uncertainty, as well as strong dependency on working conditions. Both the squeal frequencies and the sound pressure levels at all frequency levels also close to the normal distributions, with a mean of 60 dB. The statistical characteristics of brake squeals at different frequency levels are distinct from each other.