For atomic-level investigation of plasticity in amorphous metal, instability analysis which can rigorously describe unstable deformation of atomic structures is a key technique. However, the analytical method cannot be applied to large-scale amorphous metal because of the enormously-increasing computational load with respect to the number of atoms. In this study, an efficient instability analysis method for a large-scale atomic system is proposed using linear elements, and it is applied to cracked amorphous metal under tension. The proposed method successfully evaluates the instability criterion as well as the highly-localized unstable deformation. Moreover, the computational efficiency is considerably improved from the original one, enabling us to address mechanical instability problems in amorphous metal consisting of a great number of atoms.