All‐inorganic lead‐free Cs2AgBiBr6 double perovskite solar cells (PSCs) have attracted growing attention owing to their eco‐friendly features and robust intrinsic stability. However, arising from the rapid crystal growth, the poor film quality always leads to substantial non‐radiative recombination and inferior performance improvement. Herein, high‐efficiency and stable Cs2AgBiBr6 PSCs are obtained by introducing a functional polymethyl methacrylate (PMMA) layer at the perovskite surface to avoid direct contact between carbon and the underlying charge transfer layer, as well as to passivate the defects. When assembling into solar cells, the non‐radiative charge recombination is suppressed and the interfacial charge extraction is accelerated. As a result, the carbon‐electrode‐based Cs2AgBiBr6 PSC yields an enhanced efficiency of 2.25% with a high open‐circuit voltage of 1.18 V. Moreover, the unencapsulated device exhibits superior long‐term stability owing to the protection of the PMMA layer from corrosion by the extraneous water and oxygen, retaining nearly 100% of the initial efficiency after storage in 25 °C, with 5% relative humidity (RH) for 80 days and high temperature of 85 °C, and 0% RH for 60 days, respectively. A simple method of polymer passivation for enhancing the performance and stability of Pb‐free Cs2AgBiBr6 PSCs is provided.