A new type of cannon barrel with spiral grooves structure has been designed based on the principle of gas-curtain launching to reduce the launch resistance of underwater cannons. The low-resistance and high-speed launch of underwater cannons was achieved by the four spiral grooves opened on the inner wall of the barrel. A transient three-dimensional two-phase flow model of the gas-curtain launching and drainage process of underwater cannons was established， and the rationality of the model was verified. Based on this， numerical simulations of the drainage process of the spiral grooves structure were conducted， and the effects of straight grooves and spiral grooves on the gas-curtain drainage were compared and analyzed. The results show that during the initial expansion stage， the four gas jets rotate circumferentially and rapidly expand radially along the spiral grooves structure. Then， the jets interfere with each other and gradually converge， forming a columnar gas-curtain and starting· the cooperative drainage. During the formation of the gas-curtain， the spiral groove structure has a complex effect on the expansion velocity of the jet head， presenting an initial sharp decrease and subsequent decline with fluctuations. After the formation of the columnar gas-curtain， the velocity of the gas-curtain head increases nonlinearly. When the gas-curtain head expands to the outlet， the gas content of the spiral groove barrel increases by 9.3% compared to the straight groove barrel. Especially， the gas content reaches 100% within the range of 400 mm in front of the projectile.