The absorption losses of the Ag back reflector in two typical silicon solar cells have been analyzed by frequency domain finite difference method. The results show that, in flat silicon solar cell, the losses of Ag back reflector are mainly caused by intrinsic absorption and the oscillation absorption of guided modes. The absorption peaks of TM modes caused by surface plasma oscillation are larger than those of TE modes. In the textured crystalline silicon cell, the distribution of light are complex, and strong guided modes oscillation in the active layer can be introduced by normally incident light of both TE and TM modes. Strong surface plasma oscillation in Ag back reflector can be produced by TM modes. Therefore, in the textured crystalline silicon cell, the absorption spectra show multi-peak characteristics and the values of absorption peaks are larger than those in the flat type crystalline silicon cell.