Abstract: The density is one of the most important factors for the mechanical behavior of soil. The content and occurrence modes of hydrates obviously affect the density of gas hydrate-bearing sediments (GHBS) because hydrates exist in the pore of sediment as solid phase. Therefore, it is necessary to consider the effect of the hydrate content and occurrences to describe the mechanical properties of hydrate sediments well. In this paper,based on the unified hardening model for clays and sands (CSUH model), the relation between the volume fraction of hydrate and the compressive hardness parameter is firstly established to reflect the influence of hydrate on the compressibility of sediments. Secondly, in order to consider the influence of hydrate content and occurrence modes on sediment density, we propose a formula to describe the effective initial void ratio, and it is then introduced into the state parameter to describe the influence of hydrate on dilatancy of sediment. Finally, combining the drop-shaped yield surface of the CSUH model, an elastoplastic constitutive model for GHBS is developed. Compared with the laboratory test results, it is verified that the model can reasonably describe the mechanical behaviors of GHBS containing hydrates with different occurrence modes and contents. For the same occurrence mode but different contents, the set of parameters is the same.