For the new idea of a gas-curtain launcher with a grooved tube, the gas-curtain flow field and interior ballistic characteristics are mainly investigated in this paper. The coupling of the gas–liquid interaction model and interior ballistic equations is realized by solving the gas flow equation. Analyses have focused on the morphological evolution of the gas-curtain, pressure distribution, turbulence intensity evolution, and interior ballistic performance. The results show that multiple groove jets first expand independently of each other, and their shape changes from rectangular to triangular. The groove jets then come into contact with each other and form a gas-curtain. Meanwhile, the gas-curtain expansion results in complex changes in the pressure and turbulence intensity of the flow field in the tube. The parameters distribution in the flow field gradually have a simple tendency as the gas-curtain increases and the projectile moves. The moment the projectile starts moving, the gas volume fraction reaches 83%, indicating that the gas-curtain has made remarkable achievements in drag reduction. Significantly, under the calculated conditions in this paper, an initial velocity of 360.58 m/s was obtained at a maximum chamber pressure of 86.34 MPa.