In this paper, a series of low-molecular-weight PEG-PCL-PEG triblock copolymers were successfully synthesized by ring-opening polymerization method, and were characterized using 1H-NMR and FTIR. The aqueous solution displayed specific thermosensitive gel-sol transition when the concentration was above corresponding critical gel concentration (CGC). The gel-sol phase diagram was recorded using test tube-inverting method, which was depended on the hydrophilic/hydrophobic balance in macromolecular structure, as well as heating history. As a result, the gel-sol transition temperature range could be altered, which might be very useful for its application as injectable drug delivery system.