The axial modulus E_SMA(κ) of FeMnSi-based shape memory alloys (FeMnSi-SMAs) is a parameter introduced in this study to characterize the relationship between stress and strain behavior at the early stage of tensile loading. E_SMA(κ) can be used to correctly estimate and model the interaction forces between FeMnSi-SMAs and other materials. Unlike the conventional Young’s modulus, which is usually given at room temperature, the E_SMA(κ) is evaluated at different temperatures and strongly depends on phase transformation and plastic deformation. This study investigated the evolution of E_SMA(κ) during and after pre-straining as well as in the course of the activation processes. The effect of different factors (e.g., phase transformation and plastic deformation) on the magnitude of E_SMA(κ) is discussed. The result shows that the E_SMA(κ) can differ significantly during activation and thus needs to be modified when interaction forces between FeMnSi-SMAs and other substrates materials (e.g., concrete) must be modeled and evaluated.