Abstract: Improving the spectral transmittance of CdS window layer used in CdTe-HgCdTe tandem solar cells has a significant impact on the effective utilization of sunlight and increases the device short-circuit current density (Jsc). Herein, the average transmittance (T) and Jsc losses of as-deposited and CdCl2 annealed CdS films prepared by chemical bath deposition (CBD), near space sublimation (CSS) and magnetron sputtering(SPUT) are studied. The results show that:in the range of 520-820 nm, the as-deposited and annealed CdS films by CBD exhibit the highest T and the lowest Jsc losses of the CdTe top cell; in the range of 820-1 150 and 520-1 150 nm, the as-deposited and annealed CdS films by SPUT exhibit the highest T and the lowest Jsc losses of the HgCdTe bottom cell and the CdTe-HgCdTe tandem solar cells. In the range of 520-820, 820-1 150 and 520-1 150 nm, the CdCl2 annealing can greatly increase T of CdS films and decrease the Jsc losses of the CdTe top cell, the HgCdTe bottom cell and the CdTe-HgCdTe tandem solar cells.