Abstract: Aiming at the flow instability of additives in the operation of solid additive system of extruding and pelletizing unit in polypropylene plant,this paper found out the causes for the problem through analysis and verification of the process control and the additive characteristics of the system.Then it put forward corresponding improvement measures which eliminated the unstable factors of the solid additive system effectively and solved the interference of large amplitude of additive flow fluctuation to the product quality.Meanwhile,these also ensured the longterm stable operation of the system.