Mn2+ doped CdS QDSCs were obtained by successive ionic layer absorption and reaction (SILAR) method. The power conversion efficiency (PCE) of Mn/CdS device was up to 3.29% when the concentration of Mn2+ precursor solution was 0.075 M, which is much higher than that of CdS device (2.01%). We found that the Mn dopant into CdS QDs increased the light absorbance and extended the light absorption range, which resulted in the enhancement of the light harvesting efficiency (LHE). In addition, the faster speed of electron transport and longer electron diffusion length for Mn/CdS QDs devices improved the charge collection efficiency (ηcc) of the solar cells. ηcc was increased from 89.9% for CdS device to 96.7% for Mn/CdS device. As a result, the improvement of PCE for Mn/CdS QDSCs was mainly derived from the enhancement of LHE and ηc