BaSi2, which can be grown on Si(111) substrate by molecular beam epitaxy experimentally, holds great promise for solar-cells. Here, we report a detailed ab initio study on the structural and electronic properties of BaSi2(100) thin films on Si(111) substrate. A high stable interface structure with bond breaking of Si-4-tetrahedra at interface is obtained by ab initio molecular-dynamics simulations. We find that the bond breaking of Si-4-tetrahedra at interface play a key role to saturate the dangling bonds of Si(111) substrate. Electronic band structures and band-decomposed charge density distributions reveal that such BaSi2(100) thin film structures are semiconductor with an interface band gap of 0.71-0.75 eV and a large surface band gap of 1.17-1.27 eV, closing to the bulk BaSi2 band gap of 1.25 eV. These results provide an excellent explanation for the recent experimental observations on the BaSi2-based thin films on Si(111) substrate.