The invention belongs to the field of biological breeding, is specially used for Rhododendron Delavayi 'Cosmopolitan' chromosome polyploidy breeding and particularly relates to a Rhododendron Delavayi 'Cosmopolitan' chromosome doubling method. The method comprises the following steps: inducing Rhododendron Delavayi 'Cosmopolitan' explants to generate calluses, and treating the calluses with a chemical reagent colchicine to double chromosomes; transferring the calluses into a differential medium for culturing and inducing adventitious buds to generate; transferring the adventitious buds into a propagation culture medium for propagation culture; inoculating strong seedlings into a rooting culture medium and carrying out rooting culture and acclimatization and transplanting; observing the chromosomes under a microscope; and screening doubled plant germchit. The method for doubling chromosomes of an ornamental plant, Rhododendron Delavayi 'Cosmopolitan', is provided in the invention for the first time. According to the polyploidy Rhododendron Delavayi 'Cosmopolitan' cultured by the method, the ornamental value of Rhododendron Delavayi 'Cosmopolitan' is effectively improved.