In order to understand the process of direct graphene growth on the silicon carbide surface, this study employed density functional theory to analyze its growth mechanism and verifies it experimentally, and draws the following conclusions: (1) Acetylene and ethylene exhibit strong adsorption on both SiC(0001) and ( $000\overline{1}$) surfaces, with adsorption energies ranging from −5 to −2 eV. These energies enable effective adsorption and subsequent dissociation of acetylene and ethylene on the SiC surface, catalyzing the formation of active carbon species. Based on the cleavage energy barriers and the population of different carbon-active species of acetylene and ethylene, it is tentatively confirmed that CHCH is the most probable main active species in the two subsequent steps (nucleation and edge growth). (2) At low chemical potentials, CHCH exhibits a stronger nucleation tendency on the ( $000\overline{1}$) surface. In contrast, at higher chemical potentials, nucleation is more favorable on the (0001) surface. (3) The experimental results show that acetylene provides a more efficient carbon source and significantly accelerates the graphene growth; graphene has fewer defects in the conformal growth on the Si surface and more defects on the C surface.