Abstract: In large rotating machinery using filmatic bearing (turbogenerator,air compressor,etc.) there is a potential trouble from impefect installation or running conditions,a metals' contact between rotors and shells.The trouble is usully called rubbing.The advantage of AE technologe in the rubbing diagnosis and early detection is showed.Instead of classical AE characteristic parameters (counts,amplitude,energy,duration and the other ones) a new feature extraction by frequency analysis of AE envelop signals is discussed.Both research of AE process and field experements indicate that periodic components in the envelop spectrum related to rotational speed increase evidently at the beginning of rubbing production,hence are highly sensitive to the rubbing fault diagnosis.According to the new design an acoustic emission equipment for the rubbing diagnosis named BUAA AE testing system is developed and described.