SnO2-based planar perovskite solar cells (PSCs) are considered as potential photovoltaic candidates due to their simple structures and cost-effective preparation processes. However, the extensive defects accumulated at the buried interface between perovskite and SnO2 greatly hinder the further improvement of PSC efficiency and stability. Herein, the potassium salt of anthraquinone-1,8-disulfonate (ASPS) is used as a novel multifunctional interfacial modifier to improve the carrier transport performance at the buried interface and optimize the quality of the upper perovskite light absorber layer (PVK) in PSCs. Owing to the synergistic effect of sulfonic acid groups, carbonyl groups and potassium ions in ASPS, the accumulated defects at the buried interface are passivated, the energy level arrangement of the interface is optimized, and the crystalline quality and optoelectronic properties of the PVK films are improved. As a result, the power conversion efficiency (PCE) improved significantly from 21.36% for the controlled device to 23.96% for the ASPS-modified device. Furthermore, the unencapsulated ASPS-modified device also exhibited better storage stability and thermal stability than the controlled device.