The stress-strain-time relationship of the Koppejan creep model based on consolidation tests is improved to the incremental form

which is generalized to 3D stress space

and then the improved Koppejan creep model for soft soil is established in this work by the incremental form. Based on the ABAQUS software

an UMAT is developed by the improved Koppejan model. Using long-term consolidation tests of Shanghai soft clay to validate the model

the results show that the Koppejan model describes the creep property of Shanghai soft clay very well; the computational accuracy of the improved Koppejan model is better than that of the classical Koppejan model

and the two models work better for mucky clay than silty clay. Using the improved Koppejan creep constitutive model to the three-dimensional FEM of a high-speed railway bridge pile foundation long-term deformation

and comparing with the measured settlement

the results indicate that the improved Koppejan can be well used for computing long-term deformation of actual projects.