In order to study the effects of boundary constraint and separation surface width on separation performance of lead linear shaped charge， numerical simulation was used to analyze the factors affecting the separation of cast aluminum target plate after the explosion of lead linear shaped charge. The effects of free boundary constraint， fixed boundary constraint and slot width of separation surface on separation performance of lead linear shaped charge were obtained. Results show that the separation of brittle materials such as cast aluminum from lead linear shaped charge is the result of the joint action of jet penetration and high pressure detonation products. The boundary constraint has little influence on jet penetration and the deviation is generally less than 5%， but it has a great influence on the cracking process of the target plate. The strong fixed boundary constraint will limit the deformation and displacement of the material during the separation of the cutting cable， and then limit the cracking effect of high-pressure detonation products on brittle materials such as cast aluminum， which will increase the difficulty of subsequent separation. By appropriately increasing the width of the separation interface， the effect of boundary constraints on the separation performance of the cutting cable can be effectively reduced， and the separation reliability of the cutting cable can be improved. The 2.7 g·m^-1 lead linear shaped charge can reliably separate the ZL114A material with a thickness of 4.5 mm when the width of the weakening groove is ≥12 mm.