Poly(ethylene glycol) (PEG)‐based hydrogels have attracted increasing attention in recent years due to their good biocompatibility and low cost. However, the PEG‐based hydrogels prepared by traditional methods exhibit a poor machinability due to their disordered network structure. Herein, the preparation of well‐defined PEG‐based hydrogel via a facile thermally induced copper‐catalyzed azide–alkyne cycloaddition (CuAAC) reaction is demonstrated. To accomplish this, thermochemically reduced Cu(I) catalyst is adopted to trigger “click” cross‐linking, resulting in a well‐defined PEG network. The as‐synthesized PEG‐based hydrogel exhibits good mechanical performance with a tensile strength of 2.51 MPa, which is higher than the traditional PEG‐based hydrogels prepared from CuSO4/NaSac‐mediated or CuBr/ligand‐catalyzed CuAAC. Moreover, in vitro cytotoxicity and in vivo porcine subcutaneous implantation tests demonstrate that the as‐synthesized PEG‐based hydrogel has a good biocompatibility and low toxicity, making it a promising candidate for the applications in biomedical devices and tissue engineering.