Abstract: The heat generated by the electric cable is not regularly considered in the satellite thermal design. However, with the development of the thermal design, in some particular instances, the heat generated by the electric cable is found to be an important or even decisive influencing factor in the thermal design program. With the high power electrical cable in the Haiyang-2 satellite as an example, this paper compares the calculated temperature results and the in-orbit temperature data of some equipments in a cabin, and analyzes the influence of the cable's heat on the satellite's thermal design. Based on the analysis, a relationship between the heat of the electrical cable and the thermal design difference brought by it is obtained. An estimation method of the impact of the cable's heat on the equipment's temperature is proposed, together with a criterion to determine whether the cable's heat can be ignored or not in the same satellite thermal design.