We performed ab initio molecular dynamics simulations to study the initiation chemical reaction and subsequent decomposition mechanism of a 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazaisowurtzitane (TEX) crystal at 2160 K. It was found that there are three different initial reactions involved in the decomposition of an isolated TEX molecule. The activation energy barriers for the initial decomposition reactions indicate that among the three initial reaction paths, cleavage of the nitrogen–nitro bond is the easiest path to be triggered. The decomposition of the TEX crystal is triggered by the unimolecular C–H bond breaking to form a hydrogen radical. The generated H radicals can prompt other unreacted TEX molecules to decompose. Moreover, there are many multimolecular reactions during the decomposition of the TEX crystal. Overall, after the decomposition of TEX was triggered, some long chains and complicated carbon-rich heterocyclic rings were formed, and then they split to form small fragments. This study may provide useful information to understand the decomposition mechanism of cage explosives and develop new high-energy explosives.