The effective thermal conductivity of the core in a high-temperature gas-cooled pebble-bed reactor is considered one of the most important parameters for evaluating heat transfer performance. It plays a pivotal role in maintaining core integrity and ensuring reactor safety. Under the Depressurized Loss of Forced Cooling (DLOFC) accident condition, the inability to remove residual and decay heat promptly can cause the maximum temperature of the fuel elements to surpass the safety threshold of 1600 ℃. The temperature exceeds the fuel cladding limiting temperature, potentially leading to the fuel cladding rupture and the subsequent release of significant quantities of fission products, and posing a substantial safety hazard. The effective thermal conductivity model for high-temperature pebble-bed reactors remains insufficiently developed, particularly under high-temperature conditions, where heat radiation becomes the dominant mode of heat transfer. In response to this challenge, the derivation and validation of an effective thermal conductivity analytical model within the core of the reactor are carried out. Since the contribution of thermal convection to the effective thermal conductivity is less than 1%, the analytical model consists of three components: short-range and long-range thermal radiation, and heat conduction. On this basis, the analytical model of thermal conductivity was established and validated with the experimental data of the heat transfer test (Breitbach in Nucl. Technol. 49:392–399, 1980). The error margin of the analytical model is 20%, with a positive deviation of +10% and a negative deviation of −20%. The developed model was adopted to predict the thermal conductivity at high temperatures, which was 53.77 W/(mK) at 1600 K and 65.60 W/(mK) at 1800 K. In general, the effective thermal conductivity analytical model developed in this study is proven to be feasible to describe heat transfer performance inside the high-temperature gas-cooled pebble-bed reactor.