In this work, we report for the first time a simple two-step route to fabricate a novel porous metal oxide film composed of polycrystalline nanowires with ultra-small nanoparticles, good interconnectivity between nanoparticles, and a high density of ultra-fine nanopores. The as-prepared metal oxide films combine the advantages of small crystal size, high surface-to-volume ratio, and one-dimensional-nanowire-induced unique charge transport paths (with correspondingly high interconnectivity). Taking In2O3 as an example, porous In2O3 films, composed of polycrystalline In2O3 nanowires with ultra-small nanocrystals (less than 10 nm) and a high density of ultra-fine nanopores (1.6–3.1 nm), have shown very high sensitivity and good reproducibility towards ethanol gas, which are 10–20 times higher than for In2O3 octahedra and commercial SnO2 thick films. The response/recovery speeds of the as-prepared porous In2O3 films are also 5–6 times higher than for In2O3 octahedra, SnO2 nanobelts, and commercial SnO2 thick films. We believe that such metal oxide flexible films made from highly porous nanowires will replace their traditional thick film counterparts, not only in gas sensors but also in other functional devices, such as batteries, supercapacitors, solar cells, etc.