In this paper, we explored stable states in the system of 2,4,6-trinitrotoluene (TNT) crystal with a few additional hydrogen radicals (Hadd's) using a structure-search scheme based on first-principles calculations and an evolutionary algorithm (EA) to get insights into the decomposition process of TNT. We introduced three evolutionary operators acting on Hadd's and transforming only local structures of TNT molecules: "displacement", "permutation", and "mating". We searched for stable structures by increasing the number of Hadd's (n) from 1 to 2, 3, 4, 6, and 8 and constructed a convex-hull diagram for the formation energy from solid TNT and solid hydrogen. We showed that the system of n = 6 had the largest energy reduction, in which five of the eight TNT molecules in the calculation cell were transformed into NO, H2O, C2H3N, C2NO3H3, C8N2O4H7, C9N2O8H5, and C14N7O12H11. Analysis of the structural transformations observed during the EA search indicates that (1) the Hadd's approaching the TNT molecules react with C, forming a six-membered ring, and with N and O in nitro groups, leaving the TNT molecules as NO, H2O, C2H3N, and C2NO3H3, and (2) the partially decomposed TNT molecules are bonded to one another via C, N, and O.