The shape memory effect in shape memory alloys primarily arises from their unique stress plateau characteristics. However, owing to the complexity of multiple mechanisms, quantitatively analyzing the contributions of different deformation mechanisms to strain during the stress plateau stage remains challenging. This study systematically investigates the deformation evolution law of nickel–titanium shape memory alloys during the stress plateau stage by combining in situ dynamic transmission electron microscopy observations with statistical analysis of deformation defect structures in macroscopic deformed samples.  A quantitative method for calculating the contributions of various deformation mechanisms to strain is developed. The results show that prior to the onset of the stress plateau, the strain in nickel–titanium alloys primarily results from the thickening of intrinsic secondary twins. Upon entering the stress plateau region, detwinning of <011> Ⅱ-type twins becomes the primary deformation mechanism, contributing approximately 62% to the plastic deformation. With further increase in the strain, the participation of nano (001)-type secondary twins and dislocation slip gradually increases. When the strain reaches 6%, the contribution ratios of detwinning, nano (001)-type secondary twinning, and dislocation slip to plastic deformation were 69.3%, 19.3%, and 11.4%, respectively. Thus, detwinning remains the primary deformation mechanism, although the role of nano (001)-type secondary twinning is also crucial.