Ropes are critical components for mooring ships and offshore floating structures under periodic axial loads. To investigate the tensile properties and stiffness characteristics of deep-sea mooring polyester ropes, they are categorized into three stages: preloading, initial installation, and aging. Initially, the tensile properties of the ropes under static loads are examined, quantifying strain and reversible elongation rates at each stage and comparing mechanical properties of polyester ropes with those of nylon ropes. Subsequently, creep coefficient solution tests under varying tensions are designed, establishing a quasi-static stiffness empirical equation with consideration of the rope creep coefficients. Experiments have shown that the mooring ropes still undergo reversible elongation after unloading, and sufficient break-in period of the ropes can reduce their inherent deformation and increase their structural stability. Static stiffness of the ropes increases with loading time and force until it reaches a constant value. Polyester ropes have greater stiffness, smaller deformation and more stable structure than nylon ropes, making them more suitable for deep-sea mooring. The findings enable comprehensive analysis of rope stiffness evolution throughout its service life, thereby offering references for reasonable design of deep-sea taut-line mooring systems.