The invention relates to a preparation method of a polyethylene glycol grafted poly-n-butyl methacrylate amphiphilic grafted copolymer. The method comprises the steps of firstly, preparing a polyethylene glycol macromolecular initiator by using a micromolecular dyhydroxyl initiator and then utilizing a 'grafting from' strategy to prepare the amphiphilic grafted copolymer which is clear in structure and narrower in molecular weight distribution. The preparation method provided by the invention has the beneficial effects that the method is used for preparing the polyethylene glycol grafted poly-n-butyl methacrylate amphiphilic grafted copolymer which contains a hydrophilic main chain and hydrophobic side chains; the grafted copolymer synthesized by the method has the characteristics of beingadjustable in grafting density and controllable in molecular weight of chain segments of branch chains.