Abstract: Using the characteristics of G-protein coupled receptors (GPCR) for sensing specific ligands and undergoing conformational change, inserting a signal molecule in an intracellular region of the G-protein coupled receptors, converting the conformational change of the G-protein coupled receptors into an optical signal change, and detecting the presence and/or concentration of a specific ligand by means of detecting the change in the optical signal; a GPCR activated fluorescent probe (GRAB probe) is constructed according to this principle. A method for using the GRAB probe to detect a specific ligand.