We investigated the working mechanisms of CdTe solar cells in which a second absorber-layer (defined as A2) with a lower bandgap was added through SCAPS simulation. The results show various positions and bandgap values of A2 are applicable at particular parameters, and the details are analyzed. We explored the essence of the improved cell performances via adding A2 for different occasions and provided some principles and models. Some new cognition for real cells was also presented. Additionally, we studied how the parameters of the wider-gap first absorber-layer influence the cell performances and its function as 439a back highly-resistant layer.