Perovskite photovoltaics are strong potential candidates to drive low‐power off‐grid electronics for indoor applications. Compared with rigid devices, flexible perovskite devices can provide a more suitable surface for indoor small electronic devices, enabling them have a broader indoor application prospect. However, the mechanical stability of flexible perovskite photovoltaics is an urgent issue solved. Herein, a kind of 3D crosslinking agent named borax is selected to carry out grain boundary penetration treatment on perovskite film to realize full‐dimensional stress release. This strategy improves the mechanical and phase stabilities of perovskite films subjected to external forces or large temperature changes. The fabricated perovskite photovoltaics deliver a champion power conversion efficiency (PCE) of 21.63% under AM 1.5G illumination, which is the highest one to date. The merit of low trap states under weak light makes the devices present a superior indoor PCE of 31.85% under 1062 lux (LED, 2956 K), which is currently the best flexible perovskite indoor photovoltaic device. This work provides a full‐dimensional grain boundary stress release strategy for highly stable flexible perovskite indoor photovoltaics. A grain boundary stress release strategy is proposed for high‐stability flexible perovskite indoor photovoltaics by the grain boundary penetration with borax 3D stretchable molecules. The full‐dimensional grain boundary stress release enables the flexible perovskite photovoltaics deliver a champion power conversion efficiency (PCE) of 21.63% under AM 1.5G illumination and an indoor PCE of 31.85% under 1062 lux.