The increasing integration of renewable energy sources and power electronic devices has elevated harmonics disturbance as a critical issue. This phenomenon accelerates the aging, overheating, and failure of power cables and their intermediate joints. Finite element analysis software is employed in this study to systematically analyze the electrical and thermal performance of cables and accessories under harmonics disturbance. An electromagnetic-thermal coupling Multiphysics model was established to investigate the impact of various harmonics on voltage and temperature distribution across cable conductors and insulation layers. The temperature variation in cable joints under harmonic conditions was examined. The influence of contact resistance on joint temperature rise was also assessed. The results demonstrate that harmonic frequency, amplitude, and transmission characteristics collectively drive cable temperature elevation. This work provides a theoretical basis for the design, testing, and selection of cables and joints.