The invention relates to a tree-like polymer targeting nanometer drug delivery carrier. The structural general formula of the nanometer drug delivery carrier is PEG-b-PAMAM-(Cys)n. According to the invention, the carrier is sensitive to pH, and a hydrogen bond between the carrier and a drug will break when the pH of the environment is low, so the drug is released, and targeting drug delivery is realized. The carrier has an effective treatment effect on myeloma. A large amount of lactic acid generated in the myeloma cell propagation process because of glycolysis causes a case that the pH of focus positions is lower than the normal pH of a body fluid, and the osteoclast formation and activation and the enzyme catalysis bone break further reduce the pH, so complication syndromes, such as osteolytic damages and the like, are caused. The carrier has the characteristics of specificity, slow release and high drug load, and can substantially improve the treatment effects on liquid tumors, such as myeloma and the like.