In order to achieve the sustainable development of resources, the use of thermal insulation materials has become an important means of energy conservation in contemporary society. Due to the three-dimensional (3D) porous network structure, carbon nanotube sponges (CNT sponges) have excellent thermal insulation performance and thermal stability. Obviously, the experimental study on thermal conductivity of CNT sponges will provide a favorable database for the application of this new thermal insulation material, which is of great significance to meet the needs of social production. Therefore, this paper reports on a thermal diffusivity of CNT sponges at room temperature using the transient electro-thermal (TET) measurement technology, and this measurement method is specially aimed at the thermal characterization of micro-nano scale materials. By Schuetz’s thermal conductivity model, the skeletal thermal conductivity of CNT sponges is determined at 45~51 W·m-1·K-1, and the real thermal conductivity reached 0.15 W·m-1·K-1. Finally, the thermal transport mechanism of CNT sponges was explained from the perspective of micro-scale heat transfer, and the SEM images proved that the structure defect of CNT sponges would bring about the influence of phonon scattering on the sample surface and the increase of contact resistance, thus resulting in the low thermal conductivity of CNT sponges.