Most of the existing researches on cable and its joint temperature are based on the fundamental wave. With the wide application of nonlinear load in power grid, the harmonic content of transmission lines increases significantly. The influence of harmonics on the additional loss and temperature rise of cable and its joint cannot be ignored. In this paper ,taking 64/110 kV single-core cable and its joint as examples, the finite element analysis models ( FEM ) are established.Based on electromagnetic-thermal field coupling method , the influences of harmonic content and harmonic times on the cable and its joint are analyzed. Equivalent current factor is introduced to calculate the cable temperature rapidly under co-existence of multiple harmonics environment. The simulation results show that the loss and temperature of cable are related to both harmonic contents and harmonic times. Moreover, harmonics may cause overheat points at crimping clamp, which would accelerate the insulation aging. The introduction method of equivalent current factor has high accuracy in the estimation of cable temperature under co-existence of multiple harmonics environment. This paper provides a basis for the design, the prediction of operating temperature and the evaluation of thermal aging life of cables and their joints under harmonic environment.