In order to fast estimate the performance parameters of ducted solid rocket ramjet on the design point. A method of internal ballistic calculation in ramjet extended range projectiles was established, and established physical and mathematical model of ducted solid rocket ramjet, according to the theory of gas one-dimension steady flow. Using C++ language has developed a ballistic computer program within ducted solid rocket ramjet. The calculation results, by contrast with test dates, show that the method of interior ballistic calculation of solid rocket ramjet have a certain accuracy to meet the requirements of the ramjet design phase.