The invention discloses a preparation method of a fluorescent polymer having controlled light emitting. The preparation method comprises: (1) synthesizing a vinyl fluorescent monomer ODFB; and (2) adding an initiator AIBN to the vinyl fluorescent monomer ODFB as a monomer, and carrying out a polymerization reaction to obtain the fluorescent polymer PODFB, wherein the use amount of the initiator AIBN is controlled in the polymerization reaction so as to control the molecular weight of the fluorescent polymer PODFB, such that the light emitting of the fluorescent polymer is regulated. According to the present invention, the light emitting performance of the fluorescent polymer can be regulated by controlling the polymerization molecular weight, and the obtained fluorescent polymer has broad application prospects in the fields of light emitting materials, biological imaging, chemical sensors and the like.