In accordance with the twin-shear unified strength theory and the improved Nishihara model, a viscoelastic-plastic creep constitutive function of frozen soil has been established based on the associated flow rules, and the viscoelastic-plastic soft matrix is further derived from numerical calculations. The results show that the viscoelastic-plastic creep constitutive function has considerable influence on the non-linearity and the intermediate principal stress of materials, and it can describe the creep properties of frozen soil as a unified solution. The proposed model herein is applicable to the creep study of frozen soil.