The control of mercury in flue gas is challenging, especially that of elemental mercury (Hg-0). Recently, many researchers have focused on various mercury removal technologies. Here, by performing density functional theory (DFT) calculations, we systematically studied the adsorption of Hg-0 on several experimentally available hexagonal boron nitride (h-BN) nanosheets with no defects, nitrogen vacancies (V-N), boron vacancies (V-B), and both nitrogen and boron vacancies (VN + B) as well as their structures and electronic properties. Our calculation results show that the presence of V-N,V-B, and VN+B enhances the adsorption energies of Hg-0 by 9, 45, and 214 kJ/mol, respectively. Moreover, a more negative potential at the V-B and VN+B sites makes the h-BN-V-B and h-BN-VN+B surfaces more reactive than those of h-BN and h-BN-V-N. The partial density of states analysis reveals that the Hg atom interacts firmly with surface B or/and N atoms through orbital hybridization. The trend of the equilibrium constant implies that adsorption of Hg-0 on the h-BN-VN+B surface is beneficial at low temperature. Our computational studies reveal that defective h-BN nanosheets with V-B and VN + B have great potential to serve as novel sorbents for the efficient removal of mercury in flue gas.