Abstract: The effective thermal conductivity is a key parameter in the thermal design and safety analysis of the high temperature pebble bed reactor. The ZBS model is widely used to predict the effective thermal conductivity of pebble bed reactor core. In this paper, the key empirical parameter contact area fraction φ in the ZBS model was analyzed. The effective thermal conductivity in different pebble beds were numerically studied and the twelve groups of the values of φ with the corresponding contact radius ratios and coordination numbers were obtained. Then the formula for calculating contact area fraction φ was obtained through the multiple linear regression. Compared with SANA experimental data, the improved ZBS model is better than other models. Compared with previous experimental data, the improved ZBS model is able to predict the effective thermal conductivity in the bulk region of pebble bed. The results in this paper could provide theoretical guide for the design and safety analysis of the high temperature pebble bed reactor.