A series of star-shaped poly(D,L-lactic-co-glycolic acid)-b-methoxy poly(ethylene glycol) (PLGA-mPEG) block copolymers with varying PLGA/mPEG block weight ratios, mPEG block length, and arm numbers were synthesized and phase transition behaviors were investigated. Phase transition characteristics, such as critical gel concentration (CGC) and critical gel temperature (CGT), were closely related to the molecular structure of the star-shaped block copolymers. The CGC was mainly determined by the balance of hydrophobic PLGA and hydrophilic mPEG block (PLGA/mPEG block ratio). The CGTs showed a stronger dependence on mPEG block length and arm number. Also, the CGTs can be adjusted by adding mPEG homopolymer additives. The weight fraction of mPEG had a stronger influence on the CGT values than molecular weight of mPEG. In addition, the MTT assay and histological observations confirmed the acceptable biocompatibility of the star-shaped block copolymer. Hence, the star-shaped PLGA-mPEG block copolymer was a promising candidate as a novel injectable gel.