CH3NH3PbI3 (MAPbI3) perovskite materials hold considerable promise for future low cost, high-efficiency solar cells, and replacement materials for toxic lead have also been in demand. In this study, the optical constants, absorption coefficients, and interband electron transitions of MAPb1−xSnxI3 (x = 0, 0.4, 0.8, and 1) films have been analyzed by spectroscopic ellipsometry in the photon range of 1 eV–5 eV. The bandgaps of MAPb1-xSnxI3 (x = 0, 0.4, 0.8, 1) are 1.54 eV, 1.51 eV, 1.49 eV, and 1.46 eV, respectively. With the increase in Sn, the s–p antibonding coupling becomes stronger, and the bandgap energy decreases, owing to the shallower and more active lone-pair states of Sn-5s than Pb-5s near the valence band maximum (VBM). According to the x-ray diffraction patterns, doping Sn does not change the material structures, which makes the shape of VBM more fluctuating, resulting in a similar band structure. Moreover, band structures and interband electron transitions of all four samples are discussed in terms of solid-state physics and can be assigned to the direct transition between the valence band and the conduction band at R, M, and X symmetry points.