Disclosed in the present invention are a compound for performing fluorescent marking on β-amyloid plaques and/or neurofibrillary tangles, a preparation method therefor and applications thereof, a method for performing fluorescent marking on on Aβ plaques on brain slices of a transgenic mouse, a method for performing fluorescent marking on neurofibrillary tangles of brain slices of an AD patient, a pharmaceutical composition, and a synchronous detection method for Aβ plaques and neurofibrillary tangles in the transgenic mouse and a human body tissue. The present invention provides a small-molecular fluorescent compound having no charges. An electron donating group and an electron withdrawing group in the structure form a conjugated structure of donating electrons and withdrawing electrons, and a conjugated system of π-π* transition is increased by means of carbon-carbon double bonds, so that fluorescence generated by molecules of the compound moves to a long-wave direction (bioluminescence interference is reduced), and the structure wholly satisfies the affinity of Aβ plaques or neurofibrillary tangles.