As an important member of group VA–VIA semiconductors, 2D Sb2Se3 has drawn widespread attention thanks to its outstanding optoelectronic properties as compared to the bulk material. However, due to the intrinsic chain‐like crystal structure, the controllable synthesis of ultrathin 2D planar Sb2Se3 nanostructures still remains a huge challenge. Herein, for the first time, the crystal structure limitation is overcome and the successful structural evolution of 2D ultrathin Sb2Se3 flakes (as thin as 1.3 nm), by introducing a sodium‐mediated chemical vapor deposition (CVD) growth method, is realized. The formation of 2D planar geometry is mainly attributed to the preferential growth of (010) plane with the lowest formation energy. The thickness‐dependent band structure of 2D Sb2Se3 flakes shows a wide absorption band from UV to NIR region (300–1000 nm), suggesting its potential application in broadband photodetection. Strikingly, the Sb2Se3 flakes–based photodetector demonstrates excellent performance such as broadband response varying from UV to NIR region, high responsivity of 4320 mA W−1, fast response time (τrise ≈ 13.16 ms and τdecay ≈ 9.61 ms), and strong anisotropic ratio of 2.5@ 532 nm, implying promising potential application in optoelectronics. For the first time, the crystal structure limitation is overcome and the successful structural evolution of 2D ultrathin Sb2Se3 flakes (as thin as 1.3 nm), by introducing a sodium‐mediated chemical vapor deposition growth method, is realized. The Sb2Se3 flakes‐based photodetector shows a broadband photodetection range from the UV to NIR region, due to the high‐quality single‐crystalline character and 2D morphology of Sb2Se3 flakes.