In this work,deflagration to detonation(DDT) experiments of pressed PBXC03 with different confinements (shell length 420 mm, shell thickness 10 mm and 20 mm) were conducted. Electrical pins were used to measure reaction wave velocity in explosives. DDT intensity was estimated by observing shell fragments and analyzing velocity changes. According to the conditions of different high density explosive charge,a DDT calculating model was established. The explosive deflagration reactive flow model was adopted to describe explosive reaction process. The methods of separating elements and random invalidation stress were used to describe shell expansion and rupture. The effects of gas leakage and pressure reduction on explosive reaction were considered. DDT processes under different confinements (shell length 420 mm, shell thickness 5, 10, 20, 30 mm) were calculated. The results show that only high density PBXC03 with strong confinement (shell thickness 30 mm) may undergo DDT. Gas leakage and pressure reduction induced by weak confinement (shell thickness 5 mm) will restrict the occurrence of DDT.