The invention provides a fully-dissociable type 
polyethylene glycol-poly(L-glutamate-gamma-benzyl ester)-polyethyleneimine 
copolymer as well as a synthesizing method and application thereof. The method comprises the following steps: performing 
cystamine modification on terminated methoxy 
polyethylene glycol; initiating a ring-opening 
polymerization of gamma-benzyl ester-L-glutamate-N-carboxyanhydride by utilizing the terminated amine of the terminated methoxy 
polyethylene glycol; connecting the product in the ring-opening 
polymerization and polyethyleneimine by utilizing 2,2'-dithio-diethyl 
isocyanate by virtue of an 
addition reaction to obtain the target product. According to the method, the synthesized 
copolymer is amphipathic, can be self-assembled to form nano-particles; chemotherapeutic medicines and genetic medicines can be simultaneously loaded to the 
copolymer when the copolymer is used as a medical vector; the copolymer can be completely dissociated and disintegrated by responding 
glutathione and other reductive substances, so that the vector can escape from 
endosome to 
cytoplasm, medicines can be quickly released, and the 
treatment effect can be improved. The method can be used for controlling the molecular weight and length of each fragment in the segmented copolymer, is mild in reaction condition, and is suitable for more 
medicine types.