A protection-graft-deprotection method was developed to prepare chitosan-g-polycaprolactone graft copolymers, during which the ring-opening copolymerization of epsilon-caprolactone onto phthaloylchitosan (PHSC) happened without any additional catalysis. The intermediate PHCS was introduced primarily to protect the active amino group of chitosan. After controlled experiments, the phthalimido compound was proposed to be a novel kind of organic catalyst for the ring-opening polymerization of caprolactone monomers, while the hydroxyl group acted as an initiator. Hence, in this graft system, PHCS was endowed with both self-catalysis and self-initiation at the same time, and the PCL side chains grew from the hydroxyl groups of the chitosan backbone.