Abstract: Carbon fiber material has become an indispensable function material in nuclear and aerospace fields. Researches and designs of the high temperature gas-cooled reactor (HTGR) require a large amount of this kind of heat insulation material. Due to limitations of current measuring techniques, thermo-physical property data of carbon fiber material are gravely insufficient, especially for the data of high temperatures above 1000 ℃, which limits the application of this material. For this purpose, a material performance test apparatus simulating the high temperature and ambient atmosphere in the HTGR core was built by Institute of Nuclear and New Energy Technology (INET) of Tsinghua University, which can conduct material performance tests at temperatures up to 1600℃. With the measuring data from a typical experimental process in this apparatus, a complete procedure and detailed algorithm of estimating the temperature-dependent thermal conductivity of carbon insulating felt by means of inverse nonlinear heat conduction problem method were introduced. A concise algorithm of solving inverse problem based on the law of steady and transient heat conduction was proposed, which could either work alone or provide ideal initial values for other inverse problem methods. Finally, values of thermal conductivity of carbon felt used in the HTGR below 1600 ℃ were determined in the experiment, which could be used as reference in HGTR related experiments and other ultra-high temperature applications.