We study the bulk electronic states of a superconducting topological insulator, which is a promising candidate topological superconductors. Recent experiments suggest that the three-dimensional Fermi surface evolves into two-dimensional one. We show that the superconducting energy gap structure on the Fermi surface systematically changes with this evolution. It is clarified that the bulk electronic properties, such as spin-lattice relaxation rate and specific heat, depend on the shape of the Fermi surface and the type of the energy gap function. These results serve as a guide to determine the pairing symmetry of CuxBi2Se3.