The sedimentary rule of tailings slurry has an important influence on dam structure. In this paper, the sediment formation characteristics and evolution law of tailings slurry were investigated through a unidimensional sedimentation experiment. The mesoscopic structure and stratigraphic classification of tailings sediment were discussed, the relationship between morphology and time was analyzed, and the electric double-layer theory was used to explain the influence of flocculation on the sedimentary characteristics. The experimental results show that, compared with sand particles, clay particles are smaller in size, contain more clay minerals and have better adsorption, and form a high-porosity flocculent structure in a liquid environment. With respect to the changes in the mesoscopic structure, tailings sediment can be divided into a water zone, flocculation zone, subsidence zone, and consolidation zone. The process of sedimentation can be divided into subsidence and consolidation stages along the time axis and the deposition time of the clayey tailings is about twice that of sandy tailings. The sedimentation time of the sandy tailings mainly depends on the free subsidence velocity of single particles, whereas the sedimentation process of clayey tailings can be described as a function of the interface level and time. This research findings lay a theoretical foundation for correlation research of the sedimentary process and mesoscopic structure, which provide a reference for the prediction of fine-grained tailings compression deformation.