To understand the variation of their thermal conductivity, nano-porous thermal insulating materials loaded with carbon black and silica fiber were synthesized from tetraethylorthosilicate (TEOS) via a sol-gel process followed by supercritical drying, and they were characterized by thermal conductivity tester, nitrogen adsorption-desorption, SEM, laser particle size analyzer. Test results indicate that the thermal conductivity of the materials without carbon black at atmospheric pressure declines linearly and then rises linearly with increasing apparent density, and the lowest value appears at apparent density of 203 kg/m3. The variation rate of thermal conductivity in the rising region is higher than that in the declining region. With the increase of doped carbon black, the thermal conductivity of the resulting materials with the same porosity at atmospheric pressure decreases first and then increases slightly, their thermal conductivity under ultimate vacuum drops, and their gaseous thermal conductivity at atmospheric pressure grows. In semilogarithmic coordinate, the gaseous thermal conductivity reduction of the resulting material as a function of gas pressure can be divided into 3 stages according to decreasing rate, it decreases rapidly by 6 mW/(m·K) from 101.325 to 30 kPa and then decreases slowly by 2 mW/(m·K) from 30 to 0.1 kPa, while the reduction between 0.1 and 0.01 kPa can be neglected. The lowest thermal conductivity of these materials at atmospheric pressure is only 16.62 mW/(m·K) and it can be further reduced to 14.50 mW/(m·K) via doping 5 wt% carbon black into these materials.