Abstract: Taking the graphite dust in helium mainstream in the primary loop of HTR as an example,the stresses acting on the single graphite dust in the helium mainstream is preliminarily calculated and analyzed.The result indicated that the gas drag played an absolute leading role.Based on the understanding of the importance of gas drag,a simplified motion equation of graphite dust was obtained,which was used to analyze the processes of graphite dust under various conditions.The results show that under a constant mainstream velocity,graphite dust particle velocity will approach the velocity of mainstream with the law of a negative exponent;while when the mainstream velocity changed linearly with time,the speed difference between graphite dust particle and mainstream will tend to a constant value;under the helium turbine cycle in HTR,the acceleration speed of graphite dust depended on the power.