A method for the detection of weak signal in noisy background，called the signature detection，is discussed in this paper. The invariant character of poles is used for our detector. Based on the analysis of the weakness of the traditional schemes of detection，a new idea for the detection of very weak signal is presented. The basic principle of this method and the simulation results are discussed. The experiment results prove that the performance of the method is insensitive to signal-to-noise ratio (SNR).