This work is a continuation of the ref. [1, 2] and [3]. The method developed in the previous works is extended to the study of a semi-infinite n-s multi-film structure. This structure is constructed as follows. First, we obtain a crystal thin film which may be either normal or superconducting by cutting a perfect infinite crystal at two separate paralled atomic planes. And then a mixed unit O composed of a thin film A and a thin film B is formed by close metallic contact. Finally, a semi-infinite multi-film structure is constructed by placing unit O's consecutively along the direction perpendicular to the interface. The corresponding single particle Green's functions for each step have been calculated. When film A and B belong to different normal metal, the results of the numerical calculation for both the single particle density of state and the energy band of such a structure have been given. When films A and B are made of the same normal metal, the results are equivelent to that of ref [7]. Futher-more, the equations for the case that one of the thin films is superconducting have been derived. The numerical results show that 1) the whole n-s multi-film structure is superconducting, and a finite peak appears in single particle density of states near the original gap edge, 2) the whole structure is gapless just as expect.
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