The initial decomposition reactions of 1,3,5-trinitrobenzene (TNB), picric acid (PA), 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitroaniline (TNA) and 2,4,6-trinitrophenylmethylnitramine (Tetryl) were studied using ReaxFF-lg molecular dynamics simulations, and the substituent effect on the thermal decomposition behaviours of nitrobenzene compounds was evaluated through the reactant number, initial decomposition pathway, products and cluster analysis. The results show that the introduction of substituents could promote the decomposition of the reactants, increase the frequency of the nitro-nitrito isomerization reaction and intermolecular H or O atom transfer reaction, and reduce the frequency of the direct nitro dissociation reaction. Notably, these effects were most obvious in the case of TNT. Owing to the introduction of substituents, the number of hydrogen-containing products (HO2N, H2, H2O and NH3) increased. Different functional groups can also lead to variations in the quantities of decomposition products and cluster distribution. The decomposition process of the five nitrobenzenes was examined in detail through the analysis of intermediate products, revealing the distinct influence of the substituent groups. These findings contribute to an enhanced understanding of how different substituent groups influence the energy release mechanisms of energetic compounds.