Organometal halide perovskite nanocrystals have received extensive attention as a high-performance photoelectric conversion material. However, the perovskite nanocrystals prepared by the traditional batch reactor have uncontrollable size and optical properties, low utilization of raw materials, and complicated purification treatment. In this paper, a flow microreactor is designed with the assistance of finite element analysis and realized the continuous synthesis of CH3NH3PbX3 nanocrystals with a 100% yield. The average size of the prepared nanocrystalline grains is as low as 3.9 nm without any purification step. The prepared nanocrystals have narrow-band emission characteristics (the full width at half-maximum is down to 18 nm), and the quantum yield can reach 76.9%. It is also possible to prepare CH3NH3PbX3 (X = Br, I) nanocrystals covering different emission wavelengths in the visible light region by this method. The coating of a polystyrene film improved the stability of the perovskite nanocrystals. The flexible hybrid film is applied to modify a LED device, and a white LED luminescence spectrum is obtained.