The present invention relates to a thermosensitive hydrogel composition, which while comprising an amino acid ester, polyethylene glycol, and a functional group for the introduction of a functional moiety at an end thereof at a predetermined ratio, comprises a polyphosphazene-based polymer having a controlled length and a content of polyethylene glycol contained therein, in which a reversible sol-gel transition character is altered; a medical polymer hydrogel comprising the polyphosphazene-based polymer; or an ink composition for 3D printing.