Sympathetic detonation experiments of the GHL explosives in steel shell were carried out. By observing the remainder of the explosive, the deformation of the witness and steel shell, the explosive reaction state was judged and the critical distance of the sympathetic detonation was gained. A calculation model of sympathetic detonation was established. By using the non-linear finite element method, sympathetic detonation experiments of the GHL explosives in steel shell were numerically simulated by the established calculation model. In this calculation model, the method of foreordained fragments was used to describe the form of the donor’s fragments and the impact of these fragments acting on the acceptor. Numerically simulated results of the critical distances are in agreement with the experimental results. The fragments mostly in the middle part of the donor impact the acceptor, and the initiation point is in the middle part of acceptor. The steel-shell thickness affects mainly the velocity and quality of the fragments as well as the defense capability of the acceptor. Consequently the steel-shell thickness affects the critical distance of the sympathetic detonation.