In large space buildings, such as airports and railway stations, the envelope of which is dominated by glass facade, skylight and metal roof, radiant floor cooling is an effective sensible heat remove terminal for its direct longwave radiant heat exchange with high-temperature wall surface and absorption of solar radiation. The emissivity of metal and many advanced materials (such as low-e coating) range from 0.2 to 0.9, which are markedly different from the emissivity of traditional building materials (between 0.9 and 0.95). The calculation methods of the longwave radiant heat exchange in literatures were based on the precondition that the emissivity of the wall surfaces are in the range of 0.9~0.95, which are not suitable for the occasion with low-emissivity materials. In this paper, a new simple calculating method of the longwave radiant heat exchange between the radiant floor surface and indoor wall surfaces with different emissivity is proposed. Combined with the emissivity data of common materials in large space buildings, the accuracy of the new method is estimated and compared with current calculation methods in literatures, which shows that the calculating method taking the influence of emissivity into consideration is more suitable for predicting the cooling capacity of radiant floor in large space buildings.