In the heat transportation of core of high temperature gas-cooled nuclear reactor (HTGR), radiative heat transfer plays a significant role in the CFD-DEM simulations. The numerical investigation is conducted for parameter analysis and wall effect of the thermal radiation. A cell model is presented to discuss the effects of temperature and pebble size. The radiation effective conductivity is directly proportional to pebble diameter and cube of the temperature. For engineering cases, the emissivity on radiation is linear approximately. In the bulk region without wall effect, the radiative thermal conductivity is inversely proportional to the packing density. The effect of solid conductivity and gas absorption can be neglected for common gases with forced convection. With uniform continuum model and discrete particle simulation, the radiative conductivity is inversely proportional to the pebble sphericity and directly proportional to the integral of the radial distribution and radiation interaction function. And radiation characteristics in wall and near-wall region are different from that of bulk region.