Phenoxazine-based COFs (TFPB-DAPO-COF and TFPT-DAPO-COF) were synthesized and exhibited wider characteristic intramolecular charge transfer (ICT) than parent phenoxazine. Phenoxazine-based COFs can form hydrogen bonds with water molecules in aprotic solvents, weakening the ICT effect, and leading to a proportional decrease in fluorescence intensity. Therefore, DAPO-COFs can be used as a tool for the detection of water molecules. Among them, TFPT-DAPO-COF shows faster and higher selective response to water on a molecular level in THF with low detection limit (0.0656% v/v).