Acoustic Emission (AE) method proposed in this paper as a novel approach was used to evaluate residual stress in friction stir welding (FSW) of 5086 aluminum plates. A finite element method was used to evaluate residual stresses in aluminum plates caused by FSW, which was validated by the hole-drilling process. Moreover, fundamental antisymmetric Lamb wave mode (A (0)) was implemented to measure the residual stresses produced by FSW process. The novelty of this investigation is the combination of a robust signal-processing analysis and theory of acoustoelastic Lamb wave. In this analysis, an envelope of A (0) mode is used instead of signal time of flight, which obliterates the need to utilize a synchronizing clock of receiving sensors. It was shown that the duration of the signal changes linearly with residual stress variation, and a new equation was then established. Finally, it is confirmed that AE as a nondestructive approach can be an efficient tool for residual stress measurement in welded plate.