Flexible and lightweight solar cells are promising for numerous emerging applications, and organometallic halide perovskites are particularly suitable for flexible solar cell applications. Transferring state of art high efficiency perovskite solar cells (PSCs) processing onto a flexible substrate, however, is challenging, requiring substantial redevelopment of materials compositions, device architectures, and processing conditions. In this work, highly flexible, robust, stable, and high efficiency PSCs based on mica substrate have been successfully developed, exhibiting a champion photovoltaic conversion efficiency (PCE) of 18.0%, retaining more than 91.7% of the original PCE after 5000 cycles of large deformation bending, and being relatively stable against humidity under elevated temperature. Key to such excellent performance is the inorganic and transparent mica substrate that is chemically inert and stable under high temperature, so that processing developed for glass-based high efficiency perovskite solar cells (PSCs) can be readily applied without modification. Furthermore, the layered mica substrate is highly flexible, and van der Waals epitaxial growth of transparent conducting ITO relaxes the mechanical constraint on the PSCs substantially, reducing the strain in the device even under large bending deformation. We believe inorganic flexible mica substrate can be used as a platform for high efficiency PSCs compatible to glass substrate, speeding up the development of flexible PSCs substantially.