Abstract: To explore the influences on quasistatic pressure by the chemical reactions between explosion products and oxygen in constraint space after explosions, based on the energy conservation and ideal gas state equations, a formula was proposed to calculate the quasistatic pressures after explosion in constraint space by making the isobaric assumption in the detonation product expansion process. And the following analyses were carried out for the relation of the quasistatic pressure in constraint space with the chemical reactions undergone by the explosion products of TNT. The results show that for the different explosive massvolume ratios, the calculated quasistatic pressures are in good agreement with the existent experimental data. This investigation can be applied to other explosives. 