In this paper, based on density functional theory (DFT), the adsorption and cracking mechanism of CH₃OH on the Cu@C_nB₈ (n = 24, 40, 88) surface is studied. There are three possible paths of C–H, C–O and O–H for the initial cracking of CH₃OH on the surface of Cu@C_nB₈. In this paper, the adsorption energy of CH₃OH and intermediates on the Cu@C_nB₈ surface is calculated, and the related reaction energy and activation energy under the possible cracking path are calculated. The calculation results show that CH₃OH is physically adsorbed on the Cu@C_nB₈ surface. The main path of CH₃OH cracking is CH₃OH → CH₃O → CH₂O → CHO → CO. The activity of CH₃OH cracking decreases with the increase of Cu@C_nB₈ size, that is, the small size Cu@C_nB₈ can be used as an effective catalyst for the cracking of CH₃OH.