This paper is devoted to research the core flow of gun barrel with leakage. In order to built the correspondence between fired rounds and inner bore size, the comprehensive life test of gun barrel is conducted and the inner bore erosion model of gun barrel is built based on measurement data of inner bore size. Then an interior ballistic model of quasi two-phase flow with leakage is built by combining this erosion model and hydrodynamic theory. The core flow parameters of barrel with and without leakage are solved by programming respectively, the validity of the interior ballistic model and simulation results are verified by muzzle velocity test and chamber pressure test. Through analysis of the simulation results, several distribution lows of core flow in gun barrel with leakage are obtained. By fired rounds increase,the gas leakage occurs more and more early and the velocity of leaked gas gets larger, while the flow field parameters of projectile bottom all decrease.