Abstract: The code to simulate the operation history of fuel pebbles in the pebble bed high-temperature gas-cooled reactor (HTR) was developed based on Monte Carlo method. The effects of discharging burnup threshold on the average value and distribution of discharging burnup were evaluated, and the results for different core models with different pebble flow speeds were compared. The results show that the effect of discharging burnup threshold on the average value and distribution of discharging burnup is largely influenced by the span of burnup increment in each flow channel.