The first-principles calculation method is used to systematically investigate the lattice structure, energy band, density of states of the bulk Cu2ZnSnS4, surface reconstruction, and mechanism of adsorption and passivation of F, Cl and H atoms on Cu2ZnSnS4 (112) surface. We find that the surface reconstruction occurs on the Cu-Zn-Sn-terminated Cu2ZnSnS4 (112) surface and this reconstruction introduces surface self-passivation. By analyzing the partial density of states of the atoms on the S-terminated Cu2ZnSnS4 (112) surface, it can be seen that surface states near the Fermi level are mainly contributed by 3d orbitals of Cu atoms and 3p orbits of S atoms at the top of the valence band. When a single F, Cl or H atom is adsorbed on the S-terminated Cu2ZnSnS4 (112) surface, all three kinds of atoms exhibit an optimal stability at a specific top adsorption site in comparison with at the bridge, hcp and fcc sites. And this top position is also the position of the S atom that has the greatest influence on the surface states. When two atoms of the same kind are adsorbed on the surface, H, Cl or F atoms occupy the top sites of two S atoms that cause surface states on the Cu2ZnSnS4 (112) surface, which have the lowest adsorption energy. And the surface states near the Fermi level are partially reduced. Therefore, two S atoms that cause the surface states are the main targets of S-terminated Cu2ZnSnS4 (112) surface passivation. It has also been found that the passivation effect of H atom for surface states is the most significant and the effect of Cl atom is better than that of F atom. Comparing the partial density of states, the Bader charge and the differential charge of the atoms before and after adsorption, we find that the main reason for the decrease of the surface states is that the adsorption atoms obtain electrons from the S atoms, and the state density peaks of the Cu and S atoms at the Fermi level almost disappear completely. In the surface model, the F atom obtains the same number of electrons from the two S atoms, while the two S atoms have different effects on the surface states. And the H and Cl atoms obtain fewer electrons from the S atoms, that have less influence on the surface states. It may be the reason why the passivation effect of F atom is slightly less than that of H and Cl atoms.