Aiming at the problem of extracting the useful signal in the complex background of chaotic noise rapidly and accurately, the phase space reconstruction theory based on complex nonlinear system was proposed, and the method of improved Extreme Learning Machine (ELM) was used to predict the single step error and detect the weak signal. The improved K-means clustering algorithm was used to select the optimal family as training set, the improved extreme learning machine chose the weight value and the offset to improve the detection accuracy and speed. The one step prediction model of chaotic noise sequence with Lorenz system was established, and the weak target signals (including periodic signal and transient signal) that lost in the chaotic noise were detected, then the IPIX radar data of Canada Mc Master University were used, and the floater signal in sea clutter noise was extracted to do the experimental research. The results show that the method can effectively detect the very weak signal in chaos background noise, at the same time, the influence of noise was restrained to the chaotic background signal, compared with the traditional algorithms such as Radial Basis Function (RBF), the prediction accuracy is increased by 25%, the detection threshold is increased by -5 dB, the training time is reduced by 77.1 s, it has more obvious advantages in practical application.