A sodium impurity is inevitable for Cu2ZnSnS4 on a substrate of soda-lime glass during high-temperature processing. Recently, it was found that a sodium impurity could improve the photovoltaic properties of Cu2ZnSnS4-based thin film solar cells (including influencing crystallinity, affecting grain growth, increasing hole density, shifting the acceptor level closer to the conduction band, increasing carrier concentration, elongating minority carrier lifetime, and so on). Thus, sodium doping becomes an effective modification means for Cu2ZnSnS4 on the flexible substrate. However, there are some examples available in the literature that discuss the underlying physical mechanism. In the present work, the crystal structure, electronic structure, and optical properties of sodium occupying different lattice sites or interstitial sites of kesterite Cu2ZnSnS4 were systematically calculated by density functional theory within the GGA+U method. Na impurity favors occupation of the interstitial sites. If Na impurity occupies the cation lattice sites, the band gap of Cu2ZnSnS4 will be broadened, which is opposite to the situation of an Na impurity occupying the interstitial sites. The doping effects of Na in Cu2ZnSnS4 are mainly exhibited by the following aspects: energy band shifting, energy band broadening or narrowing, and effective mass of holes on the top of valence band reduction. The calculated results in the present work not only confirm experimental observationa in published articles but also provide an in-depth understanding of them. Thus, the findings could help to promote novel, high-efficiency Cu2ZnSnS4-based thin-film solar cells.