Abstract: An RNA fluorescent probe for rapidly distinguishing cancer tissue from normal tissue based on nucleolar morphological changes, the probe being (E)-1-(3-aminopropyl)-4-(2-(9-ethyl-9H-carbazol-3-yl)vinyl)pyridine-1-ium dibromide, abbreviated as CAPY-AP. The probe can target RNA in culture cells and normal tissue as well as cancer tissue and then display nucleolar morphology. The judging criteria of distinguishing the cancer tissue from the normal tissue based on the nucleolar morphological changes is only single and unconspicuous nucleolus in most cells of normal tissue, while the enlarged nucleoli and/or multiple nucleoli exist in many cells of cancer tissue. Compared with other existing RNA probes, the probe has super-high RNA affinity and super-high permeability, and can rapidly image the RNA and nucleoli in tissue sections.