Abstract: Water-entry process of parallel moving bodies widely exists in air-to-sea combat modes,such as airborne projectile elimating mines and air-dropped torpedo saturation attack, which has a strong engineering application background. In order to study the cavity evolution characteristics in the water-entry process of parallel cylinders, experimental study on the water-entry process of parallel cylinders is carried out used optical measurement method based on high-speed photography technology. The countor of cavity are identified and extracted with image processing technology. And by comparing the cavity countor between single cylinder and double cylinders with different Froude number at the time of water entry (Fr₀), the cavity evolution characteristics in the water-entry process of parallel cylinders and the effect of Fr₀ are analyzed. The experimental results show that the whole cavity shows good mirror symmetry, while the inside and outside cavity of the cylinder has obvious asymmetry. When the Fr₀ is low, the cavity closure mode is pinch off, and the closure point moves backward with the increase of Fr₀. And when Fr₀ reaches critical value, the closure mode transits to surface closure and the closure point moves forward with the increase of Fr₀ insteadly. Under the positive impact of Fr₀ and the negative impact of ambient pressure and splashing and rolling, the peak value and time length of cavity expansion and outward offset of cavity center at different depth increase first and then decrease with the increase of Fr₀. Due to the different dominant impact at different depths, the critical points of Fr₀ at which the peak and time trends change are different.