A novel analytical method with the guarded-hot-box (GHB) in investigating the thermal resistance of heat-resistant fabrics is described and the analytical method is also presented in this paper. The new apparatus is capable of measure thermal resistance of the fabrics in high temperature up to an average applied temperature of 250℃.The maximum measurement error of the apparatus is 6.5%and relative error is less than 2.8% between the introduced method and standard given value. In the GHB method, air layer thickness is the most important factor that influences measurement value of thermal resistance of heat-resistant fabrics. Results show that the method is more accurate and efficient than GB11048-89 one in measuring thermal resistance of heat-resistant fabrics.