The filling and bonding effects of hydrate increase the compactness and strength for gas hydrate-bearing sediments(GHBS), which makes the GHBS exhibiting properties similar to dense sand or cemented soil. Under the frame of unified hardening model of clay and sand (CSUH model), the mechanical properties of GHBS are summarized firstly, and a compressive hardening parameter is introduced to describe the isotropic compression characteristics of GHBS under the double influences of filling and bonding of hydrate. A bonding parameter is put forward to modify the yield function, and an evolution rule of bonding effect is also proposed. The state parameters are used to adjust the dilatancy equation to reflect the dilatancy and softening depending on density. Thus, an elastoplastic model is developed, which can describe the strength, stiffness, shear dilation and strain-softening of GHBS. The model coded and tested, and the simulation results are compared with the experimental ones of GHBS. The results show that the proposed model can well describe the stress-strain relationship, shear contraction with hardening and shear dilation with softening for GHBS.