Nanocrystalline tin (iv) oxide (SnO2) electron-transport layers (ETL) have shown great potential for achieving highly efficient, stable perovskite solar cells (PSCs), in particular low-temperature-processed flexible PSCs. Recently, studies have further shown that a modified SnO2 bottom layer facilitates the deposition of highly crystalline perovskite films, boosting the photovoltaic performance of the PSCs. The modulation of perovskite crystallization processes is a key to obtain highly crystalline and stable perovskite films; however, a fundamental understanding is still missing. Herein, we report an in situ synchrotron-based two-dimensional grazing-incidence X-ray diffraction technique to explore the SnO2 ETL-modulated perovskite crystallization kinetics for the first time. The titanium carbide (Ti3C2Tx)-MXene quantum dot-modified SnO2 (MQDs-SnO2) ETL was found to be able to rapidly induce perovskite nucleation from the precursor solution, forming an intermediate perovskite phase upon anti-solvent treatment. This substantially improves the crystal quality and phase stability of the as-fabricated perovskite film. Benefiting in addition from the superior charge extraction properties of the MQDs-SnO2 layer, a steady-state power conversion efficiency of up to 23.3%, as well as outstanding stability against humidity and light soaking was achieved for the corresponding PSCs.