The connection between offshore wind power foundation structures and steel pipe piles is usually carried out by grouting. With the development of offshore wind power to deep sea, the grouting connection between wind power foundation and steel pipe piles is deeper and deeper in underwater position. To determine the influence of seawater pressure on the basic mechanical properties of grouting material, the grouting material specimens were placed in a pressure vessel for curing, and then their basic mechanical properties such as compressive strength, flexural strength, and elastic modulus were tested. In addition, the phase composition and microstructure were analyzed by X-ray diffractometer and scanning electron microscope. The results show that the compressive strength of grouting material is improved after curing under a pressure environment. Seawater pressure does not produce a “wedging” effect, but further improves the compactness of grouting material. When the seawater pressure increases from 0 MPa to 0.75 MPa, the compressive strength, flexural strength, and elastic modulus of grouting material increase by 10.7%, 2.9%, and 16.7%, respectively. Seawater pressure does not change the phase composition of grouting material, but reduces its internal porosity and increases its compactness, thus improving its mechanical properties.