Perovskite single crystals and polycrystalline films have complementary merits and deficiencies in X‐ray detection and imaging. Herein, we report preparation of dense and smooth perovskite microcrystalline films with both merits of single crystals and polycrystalline films through polycrystal‐induced growth and hot‐pressing treatment (HPT). Utilizing polycrystalline films as seeds, multi‐inch‐sized microcrystalline films can be in situ grown on diverse substrates with maximum grain size reaching 100 μm, which endows the microcrystalline films with comparable carrier mobility‐lifetime (μτ) product as single crystals. As a result, self‐powered X‐ray detectors with impressive sensitivity of 6.1×104 μC Gyair−1 cm−2 and low detection limit of 1.5 nGyair s−1 are achieved, leading to high‐contrast X‐ray imaging at an ultra‐low dose rate of 67 nGyair s−1. Combining with the fast response speed (186 μs), this work may contribute to the development of perovskite‐based low‐dose X‐ray imaging. Dense, smooth, substrate‐integrated, large‐area and thickness‐controlled perovskite microcrystalline films are prepared. They are used as self‐powered microcrystalline X‐ray detectors with an impressive sensitivity of 6.1×104 μC Gyair−1 cm−2 and a low detection limit of 1.5 nGyair s−1, leading to high‐contrast X‐ray imaging at an ultra‐low dose rate of 67 nGyair s−1, which may further reduce the X‐ray radiation risk to patients.