The physicochemical nature of pressure dependence of the dark zone is analyzed based on the common existence of dark zone in the combustion wave of high energy nitroamine propellants. The mathematics relationship between dark zone thickness and pressure is deduced in the light of the overall chemical reaction therein. It is found that the thickness of dark zone is inversely proportional to the cube of pressure,well agreeable to Kubota's experimental results and verifying the reasonableness of the deducing method and hypothesis.