DC-bias mechanism of power transformer caused by urban mass transit system is investigated. Field audible noise and vibration measurement of power transformer is carried in DC-bias condition. The noise and vibration characteristics of power transformer before and after DC-bias are analyzed, from which the characteristic parameter of DC-bias is extracted. Time variation of the characteristic parameter is compared with that of the power transformer noise and vibration. The result shows that the occurrence of DC-bias is relevant with the regular operation process of the urban mass transit system. The signals of audible noise, vibration and direct current of the power transformer have the same variation process when DC-bias occurs. The rate of odd- to even-order harmonic of the noise and vibration signals is obviously increased compared with normal conditions. Therefore, it could be effectively applied in the DC-bias condition evaluation of power transformers.