To study the characteristics of low velocity ball water entry cavity and the effect of cavity on ball dynamic characteristics, a cavity was modeled based on the principle of independence of the cavity sections expansion. A high-speed video camera was used to observe the process of ball water entry at different velocities and the process of cavity formation. The effect of cavity formation on jet flow and dynamic characteristics of the ball as well as the cavity dynamic characteristics were analyzed. The results showed that the formed cavity had an great effect on the jet flow. The jet was sputtered in the form of a water column when the cavity was not formed. After the stable cavity was formed, the jet was sputtered around before the surface closure and sputtered upward after the surface closure. The ball acceleration changed complexly due to the changed of ball water entry velocity, the formation of cavity and the impact force. The drag coefficient decreased when the cavity was formed. The change of the cavity section area was obtained using the proposed cavity model, which was verified by the experiment results. The time of cavity formation at different depth has little difference and the cavity section has a larger maximum area at small depth.