The inferior power conversion efficiency (PCE) compared to their regular counterparts (n-i-p) and undesirable stability issues of inverted (p-i-n) perovskite solar cells (PSCs) are the foremost issues hindering their commercialization. Here, for the first time, we demonstrate a polymeric room-temperature molten salt (poly-RTMS), namely poly(1-vinyl-3-ethyl-acetate) imidazole tetrafluoroborate (PEa), as a novel type of additive to modulate the perovskite crystallization and its electronic properties. The PEa poly-RTMS containing multiple chemical anchoring sites along with strong bonding stability can firmly bond to Pb ion defects at grain boundaries and the interface of the perovskite film via coordination bond, which effectively passivates the electronic defects and enhances the photo-, thermal-, and moisture-stability of perovskite films. As a result, the PEa-modified inverted PSCs show striking performance improvements over the control with the PCE exceeding 21.4% and excellent long-term operational stability, maintaining over 92% of the initial efficiency for 1200 hours under continuous full sun illumination at 70-75 degrees C. This strategy opens a new avenue to modulate the properties of perovskites for optoelectronic applications.