A new perovskite stoichiometric alkali tin dihalides compound, Cs3Sn3F2CI7, is synthesized by a hydrothermal method. This compound belongs to the monoclinic space group of P21/c with cell parameters of alpha = 9.5645(4) angstrom, b = 14.2057(7) angstrom, c = 13.5828(6) angstrom, and beta = 93.2450(10)degrees. Unlike the common perovskites in which octahedra are interconnected to be a three-dimensional network, Cs3Sn3F2CI7 possesses a zero -dimensional structure consisting of cs(+) cations, isolated [SnCI3](-) trigonal pyramids, and dimer structural units [Sn2F2CI4](2-); the latter microscopic unit is found for the first time. The thermal stability and UV-vis NIR diffuse reflectance spectroscopy in Cs3Sn3F2CI7 are measured, and the electronic structure is calculated. Interestingly, the 5s(2) lone-pair electrons on Sn2+ cations are stereochemically active, which results in a pretty good photocatalytic activity of the title compound.