The invention discloses a fluorene-based oligomer and its synthesis method and application. The method comprises the following steps: 2-bromofluorene, 4-(2-chloroethyl)morpholine hydrochloride and potassium hydroxide are heated to reflux under the protection of an inert gas, and the reaction crude product is recrystallized in petroleum ether after silica gel column chromatography , to obtain ethyl morpholine-substituted bromofluorene monomers; ring compounds with brominated ends react with alkyl bromide, 2-isopropyl-4,4,5,5-tetramethyl-1 under palladium catalyst conditions , 3,2-dioxaborane reaction to obtain cyclic compounds with borate esters at both ends; mix the above-mentioned fluorene monomers with cyclic compounds, reflux reaction under the protection of inert gas, cool and purify to obtain fluorene-based of oligomers. The invention has good electron extraction and transport properties, effectively reduces the potential barrier between the organic active layer and the metal electrode, and can be applied to the modification of the cathode or anode of organic optoelectronic devices such as polymer electroluminescent diodes and organic solar cells Floor.