Based on the investigation of mechanical response and microstructure evolution of a commercial 7003 aluminum alloy under high-speed impact, a new simple and effective method was proposed to determine the critical strain required for the nucleation of adiabatic shear band (ASB). The deformation results of cylindrical and hat-shaped samples show that the critical strain required for ASB nucleation corresponds to the strain at the first local minimum after peak stress on the first derivative curve of true stress-true strain. The method of determining the critical strain for the nucleation of ASB through the first derivative of the flow stress curve is named the first derivative method. The proposed first derivative method is not only applicable to the 7003 aluminum alloy, but also to other metal materials, such as commercial purity titanium, WY-100 steel, and AM80 magnesium alloy. This proves that it has strong universality.