On the basis of characteristics of duplex ammunition in both motion and st ructure, the interior ballist ics process and computational assumpt ion of large caliber machine-g un duplex ammunition are described. With the aid of the interior ballist ics computat ional equat ions used in the cart ridg e, the chang ing curves of chamber pressure and velocity of both bullet s are computed. And the effect of loading factors such as the mass of bullet s, the masses‘matching, the propellant mass, the start ing pressure, and the propellant pow der density etc. on the interior ballistics characterist ics has been analyzed in detail, thus draw ing some important theories.