Introduction of dynamic thiol-alkynone double addition cross-links in a polymer network enable the formation of a self-healing injectable polymer hydrogel. A four-arm polyethylene glycol (PEG) tetra-thiol star polymer is cross-linked by a small molecule alkynone via the thiol-alkynone double adduct to generate a hydrogel network under ambient aqueous conditions (buffer pH = 7.4 or 8.2, room temperature). The mechanical properties of these hydrogels can be easily tuned by varying the concentration of polymer precursors. Through the dynamic thiol-alkynone double addition cross-link, these hydrogels are self-healing and shear thinning, as demonstrated by rheological measurements, macroscopic self-healing, and injection tests. These hydrogels can be injected through a 20G syringe needle and recover after extrusion. In addition, good cytocompatibility of these hydrogels is confirmed by cytotoxicity test. This work shows the application of the thiol-alkynone double addition dynamic covalent chemistry in the straightforward preparation of self-healing injectable hydrogels, which may find future biomedical applications such as tissue engineering and drug delivery.