The hybrid perovskite CH3NH3PbX3 (X = Cl, Br, I) is a promising material for developing novel optoelectronic devices. Due to its intrinsic non-layered crystal structure, it remains challenging to synthesize two-dimensional (2D) single-crystalline CH3NH3PbX3 with nanoscale thickness. Here, we report a bottom-up approach to fabricate large CH3NH3PbX3 2D crystals via liquid-phase growth on a mica substrate. The strong potassium-halogen interactions at the perovskite/mica interface decrease the interface energy, driving the striking in-plane growth of the perovskite. The grown 2D CH3NH3PbBr3 crystal was characterized as 8 nm in thickness and hundreds of micrometers in lateral size. Weak exciton binding energy was crucial for improving the photoelectric performance of 2D CH3NH3PbBr3. A visible-light photodetector with a metal/insulator/perovskite configuration was finally achieved with a photoresponsivity of 126 A W-1 and a bandwidth exceeding 80 kHz. Our work proves that the liquid-phase growth on mica is a controllable method to grow 2D hybrid CH3NH3PbX3 perovskites, which can facilitate both device applications and fundamental investigations.