Tin dioxide (SnO2) is widely utilized for the cost-effective electron transport layer (ETL) in perovskite solar cells (PSCs) owing to its excellent optoelectronic properties. However, the surface defect states present in SnO2 ETL prepared by conventional solution methods seriously hinder the further improvement of device photovoltaic performance. It is urgently essential to develop a facile strategy to effectively minimize the adverse effects of SnO2 ETL on PSCs. Herein, a phosphorus hetero-phenanthrene flame retardant, DOPO, is introduced as a multifunctional surface modifier to improve the interfacial properties between SnO2 ETL and perovskite. Through systematic characterization analysis, the PO group in DOPO not only passivates the defective states on the surface of SnO2 ETL, but also provides chemical chelation sites for the uncoordinated Pb2+ ions in the upper perovskite structure to improve the film crystalline quality. Eventually, the photoelectric conversion efficiency (PCE) of the optimized device is improved from an initial 19.74 %–22.57 %, along with significantly improved device stability. This work provides an important reference for the development of new multifunctional interface modification materials required for SnO2-based planar PSCs with excellent properties.