To describe the creep characteristics of frozen sand along the Qinghai-Tibet Railway, a creep constitutive model considering the coupling of stress and time was proposed. First, the tri-axial creep tests under different negative temperatures and dry density conditions were carried out. Then, based on the Nishihara model, considering the coupling influences of time and stress on the model elements, the coefficient of viscosity in viscoelastic part was modified under the effect of time and stress, and the viscoplastic element was improved. By introducing the damage variable, an improved Nishihara model for frozen sand was proposed. Finally, the new creep model was verified based on the test results. It was found that the creep curves predicted by this model agreed well with the corresponding experimental results from low stress to high stress, and the model could describe the unsteady, steady, and accelerating creep characteristics accurately under different temperatures and stress levels. Besides, the coefficient of viscosity and shear modulus in viscoelastic part and the coefficient of viscosity in viscous plastic part decreased with the increase of temperature and shear stress, while the damage variable increased. This study could provide a new choice for the creep prediction of frozen sand, and accumulate data for the study of creep theory.