Polymer-blend High Density Polyethylene (PHDPE) sheet materials have been developed in China as an alternative to the High Density Polyethylene (HDPE) materials for the manufacture of geocells. It is necessary to evaluate the creep properties of the geocell-reinforced structures to provide a reference for design of their service life and improve their application potential. In this paper, the Stepwise Isothermal Method (SIM) based accelerated creep testing, in which the test temperature was gradually increased but the load was kept constant, was employed obtain the long-term creep behavior of PHDPE and HDPE geocell sheets. Conventional creep tests were also carried out to assess the accuracy of the creep prediction. Furthermore, the Modified Burgers (MB) model and the Mon-linear Three-component (NLTC) model were proposed to predict long-term creep deformations of the PHDPE geocell sheets. The experimental results indicated that SIM creep tests could predict long-term creep deformations. The creep resistance of all three PHDPE, PHDPE-A, PHDPE-B, and PHDPE-C, was better than that of HDPE due to the addition of nylon 6 for the polymer-blending modification in HDPE. Comparison of the experimental data with the predicted data also confirmed that the MB and NLTC models could accurately model the viscoelastic-plastic creep behavior of PHDPE geocell sheets.