This paper reports an experimental investigation on the motion of a circular cylinder after it horizontally enters into a water flow with a certain slamming speed. A smart cylinder-releasing mechanism is designed to enable repeatable release of the cylinder. A high-speed camera is used to record the cylinder’s motion. The effects of three factors are investigated, including the cylinder-to-water density ratio, water speed and slamming speed of the cylinder. Data for several cases with either a solid cylinder or a hollow cylinder are obtained and compared. Variations of both horizontal and vertical displacements against these factors are presented and analyzed. It is found that the trajectories of the hollow cylinder collapse at the initial stage of submerging in water when the velocity ratios are the same.