Underwater structure is a special kind of marine equipment, which mainly works underwater, and sometimes it needs to rise freely. So making research on its rising motion and assessing its safety are an important part in the development and design of underwater structure. In the research, development and design of underwater structure, model experiment is often used to study its rising motion. However, whether the model experiment result can fully reflect the free rising motion of prototype is still inconclusive. In this paper, spheres are chosen as research objects to study the influence of scale on their rising motions by means of CFD method, and explore the relationship between the rising motions of model and prototype. Finally, a prediction method is proposed, which can be used to predict the stable rising velocity of prototype through model experiment result.