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2D semiconductors with atomically thin body thickness have attracted tremendous research interest for high-performance nanoelectronics and optoelectronics. Most of the 2D semiconductors grown by chemical vapor deposition (CVD) methods suffer from rather low carrier mobility, small single-crystal size, and instability under ambient conditions. Here, we develop an improved CVD method with controllable reverse-gas flow to realize the direct growth of quality Bi 2 O 2 Se 2D single crystals on a mica substrate. The applied reverse flow significantly suppresses the random nucleation and thus promotes the lateral size of 2D Bi 2 O 2 Se crystals up to ∼750 μm. The Bi 2 O 2 Se field-effect transistors display high-room-temperature electron mobility up to ∼1400 cm 2 ·V -1 ·s -1 and a well-defined drain current saturation. The on/off ratio of the Bi 2 O 2 Se transistor is larger than 10 7 , and the sub-threshold swing is about 90 mV·dec -1 . The responsivity, response time, and detectivity of Bi 2 O 2 Se photodetectors approach up to 60 A·W -1 , 5 ms, and 2.4 × 10 10 Jones at room temperature, respectively. Our results demonstrate large-size and high-quality Bi 2 O 2 Se grown by reverse-flow CVD as a high-performance channel material for next-generation transistors and photodetectors.