Fullerenes, represented by C₆₀ and C₇₀, are typical molecular-crystal carbon allotropes. Under high-temperature and high-pressure (HTHP) conditions, they can undergo a continuous sequence of structural evolution, including orientational ordering, initial intermolecular bonding, low-dimensional polymerization, multidimensional cross-linking, cage collapse, and amorphization. Therefore, fullerenes serve as an important bridge between molecular-crystal carbon and high-density covalent carbon networks. This review focuses on the phase transitions and polymerization behavior of fullerenes under combined pressure-temperature control. The structural features and formation mechanisms of the fcc to sc orientational transition, dimerization, typically through [2+2] cycloaddition, one-dimensional chain polymerization, two-dimensional layered polymerization, including tetragonal and rhombohedral phases, and possible three-dimensional structures are systematically summarized. The effects of different loading paths and kinetic factors on phase boundaries and product ordering are also discussed. In addition, the multi-pathway competition and disordering tendency of C₇₀ driven by molecular anisotropy are comparatively reviewed. The regulatory roles of guest species in metallofullerenes and solvated fullerenes on the polymerization pathways and physical properties of fullerenes under HTHP conditions are further discussed. Finally, based on recent progress in fullerene-derived sp³-rich superhard amorphous carbon and related novel carbon structures, potential research directions for the controllable synthesis of high-pressure carbon materials through precursor engineering and multidimensional regulation strategies are proposed.