Abstract: A model is put forward based on classical interior ballistic theory. To improve the accuracy of the model and simplify is calculation, a new concept of apparent combustion rate and relevant equations are presented. Considering the characteristics of the nitrogen gas generant and dispensers, the exponential form with a constant item as is different from the normal exponential form or linear form, adopted in the equation. Then another new parameter, representing the size of the dispenser's structure, is introduced to improve the calculation results. Finally, the model is applied to predict the dispensing velocity of submunitions in some practical tests. The results show that the calculated values agree well with the measured data.