In the operational environment of underwater vehicles, the heave motion directly influences the ventilated supercavity flow field. However, the impact mechanism of the ventilated supercavity characteristics has not been fully revealed. Therefore, this study focuses on the transient evolution characteristics of the ventilated supercavity under the heave motion of the cavitator, aiming to fill the gap in this area of research. The numerical simulations employ the volume of fluid method to capture the gas–liquid interface, coupled with a k–ε turbulence model to describe turbulent flow characteristics. The periodic heave motion conditions of the cavitator are constructed by superimposing cosine functions and using an overlapping grid technique. After the model's validity is verified against experimental data, a series of numerical simulations is conducted at heave frequencies of 10, 5, and 2.5 Hz. The results indicate that the heave motion of the cavitator significantly alters the geometry of the ventilated supercavity, causing its surface curvature to change in a regular pattern and exhibit a wave-like distribution. Under fixed heave displacement conditions, an increase in heave frequency further intensifies the wave-like fluctuation of the cavity, and the curvature along the centerline significantly increases. Mechanism analysis shows that the heave motion of the cavitator alters the vertical velocity distribution near the cavity surfaces, leading to changes in the curvature of the upper and lower surfaces of the cavity, thus inducing dynamic evolution of its overall geometry. These findings provide key theoretical support and technical reference for the structural optimization and active control of ventilated supercavities.