Provided is a thermosensitive phosphazene-based polymer including an amino acid ester moiety, a polyethyleneglycol moiety, and a moiety including a sulfate group linked directly or by a linker in a predetermined ratio, a method of preparing the same, and a hydrogel-formable composition including the same. For example, a hydrogel formed from the composition may be used for tissue regeneration or drug delivery or used as a storage, a body tissue regeneration inducer, or a filler in a body.