Interfacial engineering represents a critical step towards passivating trap states and facilitating charge transfer across interfaces in perovskite photovoltaics, thereby resulting in substantially improved device performance. Herein, we report a robust strategy for tailoring interfacial carrier dynamics via judiciously synthesized uniform perovskite CsPbX3 (X = mixed Br/I ions) quantum dots (QDs) by capitalizing on an amphiphilic star-like diblock copolymer as a nanoreactor. The composition-tunable perovskite QDs possess impressive colloidal stability due to intimate ligation with a thin layer of polymer hairs (i.e., hairy QDs). By precisely tuning the composition of CsPbX3 QDs and deliberately positioning them between a perovskite film and hole transport layer (HTL), trap states (e.g., pinholes) on the perovskite surface and at perovskite grain boundaries can be effectively passivated, and concurrently cascade energy band alignment is achieved. As such, carrier separation and interfacial hole transport are greatly facilitated, thus leading to reduced carrier transition time, prolonged life time and improved charge collection efficiency. Consequently, the resulting perovskite solar cells deliver a progressively increased power conversion efficiency up to 19.21% (with interfacial passivation using CsPbBr0.025I2.975 QDs) over the 16.15% of the control device (i.e., absence of CsPbBr3 QD-incorporation). In principle, this perovskite QD-based interfacial engineering strategy may open up new possibilities to passivate traps at interfaces and grain boundaries via convenient compositional tuning of colloidally stable hairy perovskite QDs for a wide range of high-performance optoelectronic devices.