My case is concerned with ultra high temperature, from 300 to 5*10^4 [Kelvin]. In this situation, thermal properties (heat capacity, thermal conductivity) are set as "constant" option in "Thermophysical model". Then, at any temperature range, Don't the thermal properties vary with temperature? I am not confidence at this propblem.

It's true that thermophysical properties vary with temperature, and the variation is likely to be significant over such a big range of temperature. Maybe you should try setting the properties as functions of temperatures, e.g. polynomials.