Numerical simulation was carried out to study the reaction evolution of explosive charge under weak stimulation and the effect of confinement conditions on its explosion reaction. Based on the multi-media arbitrary Lagrangian-Eulerian method and fluid-structure coupling technique， the numerical simulation of the reaction evolution growth of constrained charge and its interaction with the shell after local ignition were realized. In order to simulate the process from slow burning to violent explosion， a phenomenological model of explosion evolution growth was established. Based on the central ignition experiment of the strongly constrained spherical charge， a series of simulations were conducted and the influence of confinement conditions on the reaction pressure growth process of PBX-2 explosive was analyzed. The results show that the peak value of reaction pressure increases with the increase of shell strength or thickness. When the thickness of the steel shell increases from 5 mm to 20 mm， the peak value of the pressure increases from 163 MPa to 1357 MPa， and the process of pressure increase varies greatly. However， with the fracture and disintegration of the shell， the transformation from explosion to detonation of the explosive charge is restrained.