With increasing offshore engineering projects, marine sand can be used as an economic building material, but there is no relevant literature on the shear characteristics of marine sand under higher stresses. In this paper, the direct shear characteristics of medium-sized marine sand under higher vertical stresses are studied, and the influence of shear rate on the strength index is explored. With the increase of vertical stress, the softening characteristics of the stress-displacement curve of marine sand become more and more prominent. The displacement when the marine sand reaches the peak stress tends to increase with the increase of vertical stress. During the shear process, the vertical displacement first decreases gradually and then tends to become stable, showing shear expansion characteristics. The relationship curve of vertical displacement and shear displacement presents a slight shear contraction process in the initial process, representing a long-term stable period mainly due to material selection and high vertical stress. The shear rate has a certain impact on the slope of the Mohr-Coulomb curve under the action of high vertical stress, which can be roughly understood as the increase of shear rate is represented as the decrease of internal friction. Our study results have some values for offshore engineering projects.