In this work, a series of biodegradable triblock poly(ethylene glycol)-poly(epsilon-caprolactone)-poly(ethylene glycol) (PEG-PCL-PEG, PECE) copolymers were successfully synthesized by ring-opening copolymerization, and were characterized by (1)H NMR, FT-IR, GPC, and DSC. Aqueous solutions of PECE copolymers underwent thermosensitive sol-gel-sol transition as temperature increases when the concentration was above corresponding critical gel concentration (CGC). Sol-gel-sol phase transition diagrams were recorded using test tube inverting method, which depended on hydrophilic/hydrophobic balance in macromolecular structure, as well as some other factors, including topology of triblock copolymers and solution composition of the hydrogel. As a result, the sol-gel-sol transition temperature range could be varied, which might be very useful for its application as injectable drug delivery systems. The in vivo gel formation and degradation behavior was conducted by injecting aqueous PECE solution into KunMing mice subcutaneously. In vitro degradation behavior, in vitro drug release behavior, and cytotoxicity were also investigated in this paper. Therefore, owing to great thermosensitivity and biodegradability of these copolymers, PECE hydrogel is believed to be promising for in situ gel-forming controlled drug delivery system.