To study the stability of a cast polymer-bonded explosive （PBX） during penetration， projectiles with the PBX-1 charge was launched by a 155mm gun to penetrate concrete targets. The penetration depths at different velocities were obtained， and the critical penetration velocity for maintaining the charge stability was about 490 m·s^-1. Based on the viscoelastic statistical crack （Visco-SCRAM） model， the experiments for testing penetration stability were modelled by a fluid-solid coupling method with large deformation. Thermal-mechanical responses of the charge during penetration were obtained and reasons for the unexpected ignition of the charge were analyzed. The numerical results and experimental data are in a good agreement. The results show that no obvious deformation or damage is observed for the projectile shell， while the cast PBX charge undergoes large deformation and part of charge seeps out from the end gap. During the penetration process， the charge collides with the bottom of charge chamber at high speed and forms a local high-pressure zone. The maximum pressure exceeds 500 MPa when the charge tail is impacted. The tail of the charge is deformed and damaged severely， which may lead to a highly localized temperature and even ignition.