Abstract: In the study of differential electromagnetic attack, alignment processing is the key to the attack success.In trace alignment process, the method of Taylor series expansion is proposed to get noise offset on the base of phase correlation function.By the method of maxima extraction technology of aligned data, the dimensionality reduction of electromagnetic leakage curve is achieved, and the attack complexity is reduced.A new criterion is proposed to detect the alignment effect by Euclidean distance.Experiment results demonstrate that the fluctuation degree of the unaligned trace.Euclidean distance is obvious with a larger margin than the aligned trace within certain dimensions.The degree of trace alignment can be determined by the Euclidean distance fluctuations level.