In order to analyze and judge the power transformer core looseness fault，the vibration signal of transformer tank surface is studied.A fault diagnosis method for core looseness is proposed based on the transformer tank surface vibration signal.Firstly，the vibration characteristics of the transformer core are analyzed and 100Hz is taken as the characteristic frequency in the core looseness vibration signal.In order to avoid the influence of the natural frequency of the transformer，tank wall resonance principle is studied.The natural frequency of the transformer tank surface is determined by using excitation method so as to avoid the impact of the natural vibration frequency on fault diagnosis.Finally，a method with the ratio between 100Hz characteristic frequency component and the total component（within 1kHz）to determine the core looseness is proposed.When transformer core looses，comparing the vibration signal of each measuring point longitudinally（compared with the historical data）with proportional threshold method can determine whether the transformer core has looseness fault.And comparing the measuring point horizontally can preliminary determine the transformer core loosening fault location.The method is simple and effective，and do not have to consider the change of voltage as well as the type difference of sensor to the fault judgment.