The present invention discloses one kind of superbranched Si-base macromolecular photoinitiator and its preparation, and features that the superbranched Si-base macromolecular photoinitiator is prepared through the first Si-H addition reaction to prepare superbranched organosilicon polymer with excellent flowability, high solubility and excellent reaction performance, and the subsequent reaction of the organosilicon polymer and modified small molecular weight photoinitiator. The superbranched Si-base macromolecular photoinitiator can initiate the polymerization or cross-linking of compound containing acrylate or similar radical under the irradiation of ultraviolet ray. It is superior to small molecular weight photoinitiator, and has low mobility, low toxicity, high heat resistance and capacity of raising the heat resistance of the cured resin.