In this paper, a new configuration of an earthworm-like robot with Yoshimura-origami structure and artificial muscle fiber actuation is proposed. The artificial muscle fiber is made of silver-plated nylon wire, which realizes its contraction by electric heating. It is a typical electro-thermo-mechanical coupling material. However, the electro-thermo-mechanical coupling characteristics of the fiber are not clear. To this end, based on the first principle, the differential model between the driving current and the fiber temperature under isometric contraction is proposed first. Then, the parameters of the model are identified using the NSGA-Ⅱ algorithm. Further, based on the data-driven modeling method, the relationship between the fiber temperature and contraction force under isometric contraction and the relationship between the fiber deformation and contraction force under isothermal contraction are clarified. Finally, the accurate description of the electro-thermo-mechanical coupling characteristics of the artificial muscle fiber is realized, which lay the foundation for the analysis and optimization of the motion performance of the new earthworm-like robot.