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The semi-sealed cylindrical shell is a hollow cylinder, of which one end is opened and the other end is sealed. In this paper, the hydrodynamic characteristics of a semi-sealed cylindrical shell at high-speed are studied experimentally. The influence of the slenderness and Froude number on the expansion characteristics of the cavity is preliminarily studied, with special attention paid to the cavity evolution, splash and kinetics of the cylindrical shell under the influence of the internal jet. The results show that, with the semi-sealed cylindrical shell penetrating into water, it forms supercavity on the outer shell surface similar to a solid cylinder. Subsequently, with the internal jet poured out the shell, a foggy cavity is formed below. As the internal jet continuously pours out of the shell, a new water surface is formed below the shell and the shell will impact on the new water surface, thus causing new cavities generated. This process will repeat continuously as the shell moves downward. Moreover, the slenderness of the shell and Froude number have remarkable influence on the motion characteristics and cavity evolution. The findings in this paper can be used as a reference for future designs of cylindrical shell projectile for water entry. • The motion mechanism of semi-sealed cylindrical shell during the vertical water entry is fully studied. • The shell structure can change significantly the evolution of the cavity shape. • A internal jet is formed upwards the shell and the free surface seal mode can be altered. • Comparison between different Froude number and slenderness ratio of semi-sealed cylindrical shell are investigated. [ABSTRACT FROM AUTHOR]