Abstract: Through 90 days of creep tests, the creep laws of composite geomembrane at different thickness levels are compared and analyzed. The previous typical creep models are classified and compared with the creep test results. An empirical creep model based on thickness level is proposed to verify its applicability and reliability. Based on the analysis of materials and creep data, a three-parameter viscoelastic constitutive model is proposed, and its rationality is demonstrated by using the Marquardt optimization iterative algorithm and the global optimization to solve model parameters. The results show that the composite geomembrane exhibits strong lateral contraction under the long-term tensile action of the load. The instantaneous displacement of the load increases rapidly, and the strain increment decreases with the increase of time, and finally tends to be stable. The strain of different film thicknesses is little affected by time. When the load level is 60%, every 0.1 mm increase in film thickness from 0.4 mm to 0.9 mm reduces the strain by about 6% in long-term stability. The empirical creep model based on thickness can effectively reflect the relationship between creep strain and time of two films, and the initial stage has better fitting effect. The three-parameter viscoelastic constitutive model is suitable for the attenuation creep curve and can accurately reflect the final strain of the materials in the creep stage.