The method of Transient Electromagnetic(TEM) detection is widely applied to the field of coal mine and oil exploration. But since Transient Electromagnetic signal is a non-stationary , nonlinear , weak signal under the background of heavy noise , traditional method which is based on the signal superposition is only suited to the signal of the superficial layer , it can not fetch the weak signal of the deep layer. noise reduction of the transient electromagnetic weak signal under heavy noise based on power detection of EMD decomposition is presented in this paper. This method does not need prior information. Through the method, the transient electromagnetic weak signal under heavy noise is decomposed to several intrinsic mode functions. Then by power detection, the noise leading component is extracted and reduced .At last the transient electromagnetic weak signal is obtained adaptively. The example of this paper show: when the received TEM signal of deep layer is -30db SNR, This method can reduce the most of noise, obtain the transient electromagnetic weak signal which contains geological information, and improve the depth and accuracy of the TEM detection.