The bottom-up growth mechanism of fullerenes in the range of C60-C70 has been studied step by step, and a novel growth pathway was put forward for the first time. Analysis on geometrical connectivity indicates that almost all of the synthesized pristine C60-C70 fullerenes participate in the growth channel, and those molecules are linked by simple and successive C2 insertion reactions, implying that the formation of fullerene is not a random but certain event and those experimental findings are not discrete but related to each other. In energy, except for one step of unavoidable isomerization from C68 to C70, density functional theory calculations indicate that the growth processes are exothermic with low energy barriers (<3 eV). Kinetically, the reaction rate for each step was estimated, and the results demonstrate that addition reactions easily to occur at elevated temperatures.