The interactions within co-carbonization systems are of great importance in the production of premium cokes. In this study, the blending of the heaviest fluidized catalytic cracking slurry oil fraction and the rest fractions was investigated in terms of colloidal stability, hydrogen transfer ability, and its synergistic effect on subsequent carbonization. The colloidal stability determined by spot test shows that a smaller difference in solubility parameters between two blending fractions leads to a higher colloidal stability of the blended fraction. The hydrogen transfer ability of the blended fractions determined by a previously proposed method reveals that there exists a synergistic effect between two blending fractions, and when the blending fractions have closer boiling range and higher hydroaromatics contents, the blended fraction has a higher synergistic index of the hydrogen transfer ability. The blended fraction having the highest synergistic index of hydrogen transfer ability also produces semicokes with the best quality in the subsequent carbonization process as revealed by the synergistic index of optical texture index. The close relationship between the carbonization performance and the hydrogen transfer ability of the carbonization feedstocks was further validated in this study. Guidelines for suitable selection of blending feedstocks to obtain better cokes are also provided.