The invention discloses a benzocarbazole compound and a preparation method and an application thereof. The compound is used as a solar energy dye sensitizer. A synthetic method of the benzocarbazole compound is characterized in that a compound as a formula IV is mixed with cyanoacetic acid and an organic solvent, under effect of piperidine and protection of nitrogen, carrying out heating reflux, stirring and reacting the materials for 8-12 h, after a reaction is completed, performing spin drying on a solvent, and performing silica gel column chromatography on residues by an eluant solvent to obtain the benzocarbazole solar energy dye sensitizer shown as a formula I, a formula II or a formula III. The benzocarbazole compound can be used as the dye sensitizer for a dye sensitization solar battery, and adds a novel applicable substance for screening the dye sensitizer.