To improve the interfacial charge transfer that is crucial to the performance of perovskite solar cells, the interface engineering in a device should be rationally designed. Here we have developed an interface engineering method to tune the photovoltaic performance of planar-heterojunction perovskite solar cells by incorporating MAPbBr(3-x)I(x) (MA = CH3NH3) quantum dots (QDs) between the MAPbI(3) perovskite film and the hole-transporting material (HTM) layer. By adjustment of the Br:I ratio, the as-synthesized MAPbBr(3-x)I(x) QDs show tunable fluorescence and band edge positions. When the valence band (VB) edge of MAPbBr(3-x)I(x) QDs is located below that of the MAPbI(3) perovskite, the hole transfer from the MAPbI(3) perovskite film to the HTM layer is hindered, and hence, the power conversion efficiency decreases. In contrast, when the VB edge of MAPbBr(3-x)I(x) QDs is located between the VB edge of the MAPbI(3) perovskite film and the highest occupied molecular orbital of the HTM layer, the hole transfer from the MAPbI(3) perovskite film to the HTM layer is well-facilitated, resulting in significant improvements in the fill factor, short-circuit photocurrent, and power conversion efficiency.