The ordinary shaped charge has a low kill-radius because of its structure. In order to adapt to the different battlefields, this paper proposed the double-layer sub-caliber shaped charge, which is modified from an ordinary shaped charge with insensitive explosive appended to its outer layer. In this paper, a study of the Ø56 mm shaped charge was conducted as a benchmark case. The simulations were carried out using AUTODYN for jet formation and penetration of different axial thicknesses of additional insensitive explosive and the additional height of the charge, with the aim to obtain the optimal size of double-layer sub-caliber shaped charge. The results show that the optimum axial thickness of additional insensitive explosive is ΔR/R0 = 0.8, and the optimum size of the additional height of charge is ΔL/L0 = 0.4.