A dual temperature thermonuclear burn model is presented， based on which the variation of the particle temperature， effective radiation temperature and the particle number density has been calculated for burn processes under various initial D-T density conditions. In comparison with the approximation that alpha particles deposit their energy instantaneously， the alpha particle slowing down effect during ignition has been studied. It was found that the peak of particle temperature delays and was about 13keV lower than that in the instantaneous case when the alpha particle slowing down effect was considered. Calculation also showed that in the case of lower initial temperature and density， the alpha particle slowing down effect is more remarkable.