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The Cu-doping effects on dehydrogenation of MgH2 (001) and (110) surfaces were investigated by using first-principles calculations. On the basis of the calculation results of total energy, the Cu dopant prefers to occupy the interstitial site on both surfaces rather than substitute a Mg atom, and the Cu dopant bonds with four adjacent H ions to form a CuH4 cluster. The electronic structures show that Cu–Mg and Cu–H interactions strongly weaken Mg–H interactions, which leads to a reasonable expectation of decreased dehydrogenation temperature. The kinetic barriers for Cu-doped (001) and (110) surfaces are, respectively, reduced to 1.83 and 1.48 eV, showing that improved kinetics can be expected due to the much lower desorption barriers on both Cu-doped surfaces. The present results are beneficial to resolve the disputes between previous reports on the catalytic effects of Cu-doping in the MgH2 system.