Abstract: Three kinds of relationships between characteristic parameters of gas radiation and mean beam lengths were found by using exponential wide band model, mean beam length calculation and Beer law. The emissivity, absorption coefficient and corresponding mean beam length in a non-gray gas system were obtained by coupling calculation, thus avoiding the absorption coefficient indispensable to inversely calculate a mean beam length. The results showed that compared with Hottel's chart and formula for mean beam lengths, the method presented in this paper is accurate enough to solve the problem of radiative heat transfer in engineering. Taking a zone of ternary reheating furnace model as an example, the emissivity and mean beam length for the slab and wall surfaces are calculated, which provides necessary conditions of calculating radiative heat transfer.