The envelope analysis, which is very effective in the fault diagnosis of reciprocating machinery and vibration machinery, analyzes the process of depicting the envelope, so that the computer can automatically grasp the feature points of the envelope, on this basis, after de-averaging, smooth unequal distance interpolation and other processing step by step decomposition of the fitting signal, it can well extract the envelope signal of each order, which provides a powerful analysis tool for fault diagnosis. After the analysis of analog signal and engineering signal, it is proved that the proposed method is effective and feasible.