Disclosed are a monofunctional branched polyethyleneglycol and a bio-related substance modified by same. The general formula of the monofunctional branched polyethyleneglycol is shown in Formula (1), and the general formula of the bio-related substance modified by the polyethyleneglycol is shown in Formula (2), wherein X 1 and X 2 are respectively an alkyl group with 1 to 20 carbon atoms n 1 and n 2 are respectively an integer ranging from 1 to 1000 n 3 is an integer ranging from 11 to 1000 L 1 and L 2 are respectively a linking group stably existing in illumination, enzyme, acidic, or alkaline conditions p is 0 or 1, and q is 0 or 1 R1 is a hydrogen atom or an alkyl group with 1 to 20 carbon atoms D is the bio-related substance and Z is a linking group, through which a functional group capable of reacting with the bio-related substance is linked to a symmetric axis polyethyleneglycol main chain. The bio-related substance modified by the polyethyleneglycol maintains good biological activity, and has good solubility and long metabolic half-life in an organism.
