The invention discloses a cationic brush block copolymer and a preparation method as well as an application of the block copolymer. The cationic brush block copolymer of which a side chain contains biodegradable polyphosphonate is prepared by combining an ATRP (atom transfer radical polymerization) with an ROP (ring-opening polymerization) for the first time; and the preparation method comprises the following steps of: firstly, carrying out ATRP polymerization reaction on hydroxyethyl methacrylate by a micromolecle ATRP initiator, thus obtaining macromolecule with bromine or chlorine substituent at the tail end; carrying out ATRP polymerization reaction between the macromolecule serving as a macromolecular ATRP initiator and a monomer sensitive to PH to obtain a PH response diblock copolymer; and carrying out ring-opening and graft copolymerization on a ring phosphate monomer by using hydroxyl on the side group of the diblock copolymer, thus obtaining the cationic brush block copolymer of which the side chain contains polyphosphonate. The novel cationic brush block copolymer of which the side chain contains polyphosphonate has good biocompatibility and biodegradability and low toxicity, so that the block copolymer can be used as a gene vector, and has good application prospect in a biomedicine.