Abstract: The pebble bed effective thermal conductivity represents the overall heat transfer capability of the high temperature pebble bed reactor. Improving the precision of the effective thermal conductivity model is very important to the thermal design and safety analysis of the high temperature pebble bed reactor. In order to optimize the effective thermal conductivity model of the pebble bed near the wall region, the three-dimensional and steady heat transfers combining the conduction and radiation in the random pebble beds were numerically solved. The random packing pebble bed was generated based on the discrete element method. The porosity distribution, the coordination number and the contact angle of the random pebble bed were varified by the correlations. The surface-to-surface radiation model was adopted to calculate the radiation heat transfer in the pebble bed. The spatial distribution of local effective thermal conductivity was carefully analyzed. Based on the effective thermal conductivity distributions, the largest gradient occurs at the wall region within a distance about 1dp away from the wall, becasue the local solid fraction in the wall region varies rapidly. The bulk region can be defined in the range of 4dp-11dp, in which the local effective thermal conductivities varies mainly due to the temperature. It was found that the wall-region effective thermal conductivity is significantly lower by about 22% than the effective thermal conductivity in the bulk region and the near-wall region. The reason is that the local solid fraction is low, which would weaken the conduction heat transfer as well as the radiation heat transfer. The previous improved ZBS model has a good prediction ability in the bulk region and the near wall region of the pebble bed, but needs to be further improved in the wall region. Therefore, the correction coefficient C_w is introduced to the previous improved ZBS model to better predict the effective thermal conductivity in the wall region. The correction coefficient C_w=1 for the bulk region and the near-wall region, and C_w=0.78 for the wall region. Compared with the previous experimental data and the HTTU experimental data, the optimized ZBS model is proved to be capable to predict the effective thermal conductivity in the wall region. It is to note that the correction model is suitable for the conditions when the temperature is below 1 600 K. The results in this paper could provide theoretical guide for the thermal design and safety analysis of the high temperature pebble bed reactor.