Thermal drag in duct flow under heating is studied on the background of tunnel fire.Numerical simulation of pressure field directly proves the existence of thermal drag and indicates that the value of traditional thermal drag is much larger than the real loss of energy.Based on energy equation

it is proposed that thermal drag in non-viscous horizontal duct should be equal to total pressure loss of air flow passing through heating zone

and not equal to static pressure loss.The value of revised thermal drag is half of the traditional value in uniform cross-section duct.The revised concept of thermal drag is endowed with strict meaning of fluid mechanics.By introducing a new thermal drag coefficient

formulas for two thermal drag coefficients are deduced and an approximate formula for thermal drag under small Mach number is estabilished.Consistency between the formula and equation of stagnation pressure loss is showed by comparison

and the formula is proved a accurate and practical method under low mach.The results develop the theory of thermal drag in duct flow and it is of practical value to be applied to engineering thermophysics.