The stability of Sn-based perovskites has always been the main obstacle to their application. Interface engineering is a very effective method for improving the stability of perovskites and the efficiency of batteries. Two-dimensional (2D) monolayer SnS is selected as a surface-covering layer for the CsSnI3 lead-free perovskite. The structure, electronic properties, and stability of the CsSnI3-SnS heterostructure are studied using density functional theory. Due to the different contact interfaces (SnI2 and CsI interfaces) of CsSnI3, the interface electronic-transmission characteristics are inconsistent in the CsSnI3-SnS heterostructure. Because of the difference in work functions, electrons flow at the interface of the heterostructure, forming a built-in electric field. The heterostructures form a type-I energy-level arrangement. Under the action of an electric field in the CsI-SnS heterostructure, electrons at the CsI interface recombine with holes at the SnS interface; however, the holes of the SnI2 interface and the electrons of the SnS interface are easily recombined in the SnI2-SnS heterostructure. Moreover, monolayer SnS can enhance the light absorption of the CsSnI3-SnS heterostructure. Monolayer SnS can inhibit the migration of iodine ions and effectively improve the structural stability of the SnI2-SnS interface heterostructure. This work provides a new theoretical basis for improving the stability of lead-free perovskites.