Carbon nanotube (CNT)fibers are inevitably subject to severe plastic deformation under complex loading conditions while applying in wearable electronics. In this work, we uncover the time-dependent structure evolution mechanism of twisted CNT fibers by a series of continuous and discontinuous loading experiments. The results reveal that the highly twisted CNT fibers are more sensitive to strain rate and have large stress attenuation during the relaxation process. The in-situ tensile test under scanning electron microscope (SEM)indicates that the twisting angles of CNT fiber decreased during the stretching and relaxation due to the slippage and rearrangement of CNTs. An unsteady ternary model is developed to describe the stress relaxation behavior of twisted CNT fibers taking strain rate effect and the time dependence of viscosity into account; then the constitutive equations are mathematically derived and analyzed. We believe that this work would provide beneficial guidance for designing twisted CNT fiber with high reliability and durability.