Abstract: Using optical structure analysis as the main characterization methods, the course of shot coke formation and the effects of feedstock properties and operating conditions were investigated in a small scale experiment equipment. After analysis of reasons for shot coke formation, the corresponding suppression measures were suggested. The results show that the course of shot coke formation can be expressed as feedstock → unstable spheres of mesophase → mosaic mesophase → shot coke. When the ratio of asphaltene to carbon residue is higher than 0.5, the atomic ratio of H/C is below 1.5 and the colloidal stability parameter is less than 3.5, the shot coke is easily formed. Various circulation fractions play different roles in inhibition of shot coke formation. When using heavy coker gas oil (HCGO) (420~500℃) as circulating fraction the best effect is obtained. The shot coke formation can be inhibited by optimizing process parameters like adoption of lower temperature, higher pressure, higher recycle ratio and adding tetrahydronaphthalene or fluid catalytic cracking (FCC) slurry to the reaction system.