Abstract: In the process of the high speed water entry of a conical cylindrical projectile at small angle, the initial cavity develops asymmetrically. With the decrease of the water entry angle, the asymmetric phenomenon of the development of the initial cavity is intensified. As a result, the projectile body is subjected to the step abrupt moment, resulting in a large change in its attitude angle, which seriously affects the stability of the projectile's water-entering trajectory, and even the phenomenon of water-entering ricochet occurs. In order to improve the ballistic stability of high-speed projectile during small angle water entry, a stepped cylindrical projectile design scheme is proposed based on the principle of cavitation effect of cavitator. Based on fluid volume multiphase flow model and dynamic mesh technique, the numerical calculation method of the small angle water entry of the supercavitation projectile is established, and the effectiveness of the numerical method is verified by water entry experiments. By comparing the calculation results of the stepped cylindrical shape model and the conical cylindrical shape model, the influence of the cavity evolution characteristics of different projectile shapes on the hydrodynamic characteristics and ballistic stability is obtained. The results show that the shape of the stepped cylinder can accelerate the development of the primary cavity, and there is a phenomenon of multi-cavity fusion. When the angle of attack is 0°, the cavity size does not change after the cavity is fully developed. Under the condition of small angle of attack (5°), the area of the cavity-wrapped body increases, which improves the lift performance of the projectile. In the process of the water entry with small water entry angle, the development of the cavity form in the cone of the projectile has an important effect on the water entry stability. The stepped cylinder shape can effectively accelerate the development of water entry cavity, form a recovery moment to effectively restrain the continuous increase of the angle of attack, and improve the initial ballistic stability of the high-speed projectile in the small water entry angle.