Abstract: [Objectives] TNT (Trinitrotoluene) equivalency has been a widely accepted concept in engineering. However, as a fuel-rich explosive, when TNT is detonated in a confined space, the burning effect of the blast products is considerable, resulting in an increased blast load on the chamber, so that the TNT equivalent method in a free field is no longer applicable in the confined condition. [Methods] In this paper, in order to reveal the influence of afterburning effect of detonation products under the condition of confined blast, a series of TNT detonation tests in a confined chamber filled with two different atmospheres, air and nitrogen were performed. The temperature, pressure-time histories and deflection of blast loaded steel plate were recorded. [Results] It was found that the pressure, temperature and dynamic response of the target plate in the confined space was significantly lower in the nitrogen environment than in the air environment due to the difference in the amount of energy released from the combustion of the burst products. [Conclusions] The relationship between the deformation of the target plate and the energy released by the explosive was established. The TNT equivalent method, which takes into account the afterburning effect in confined space explosions, was proposed and applied to determine the TNT equivalent of the cased charge. The validity and applicability of the method is illustrated by comparing it with the existing TNT equivalent method based on quasi-static pressure.