Using the new inhomogeneous medium interface equivalent parameter scheme combined with the finite-difference frequency-domain method, numerical analysis on mechanism of the optical properties of the thin-film solar cell with periodic structure was presented. The plasmon resonance condition was used to study the reflection, the transmission, the absorption characteristics, and the distribution of total field. In the low frequence point, the enhancement of the total field is obviousat the interface of the medium layer and the Ag layer, and the absorption increases significantly. Furthermore, the relationship between the enhancement factor, the structure, the incident wavelength and the angle of incidence was discussed, and its intrinsic physical mechanism was also analyzed. The enhancement factor is very large at low frequency and increases with the enhancement of the incident angle. The results show that the enhancement factor can increaseto 5.7. The proposed research work can provide a relevant theory and technical reference for the design and optimization of the actual organic thin-film solar cells.