A method of preparing film silicon with variable optical gap is introduced, and the relevant results are given. An easy way to determine the film optical gap by transmittivity curve tangent is shown, and the preparation craftwork and conditions are given. The gap state distribution maps of various materials are presented. In experiment, it is found that the optical gap width of material is related not only to the quantum size effect, but also to the height and width of the barrier formed by lacuna, and to the length of short range order (atom cluster length) as well. A relationship between optical gap of silicon film and atom cluster length is given. Computations show that electron liquid level lifts in potential well with atom cluster length increasing, and the scattering of carrier is weakened by defect barrier. When the atom cluster length is shorter, the electron liquid level does not always lift with atom cluster length increasing, but fluctuates heavily and forms an indention wave. The computations also show that in the case of a constant ratio of the barrier width to the length of the atomic string, the electronic liquid level is also related to the barrier height.