Disclosed are in situ-forming injectable hydrogel and medical uses thereof. In the in situ-forming injectable hydrogel two or more homogeneous or heterogeneous polymers are bonded to each other by a dehydrogenation reaction between phenol or aniline moieties on adjacent polymers, wherein a polymer backbone is grafted with a phenol or aniline moiety using a linker. In contrast to conventional hydrogel, the in situ-forming injectable hydrogel is superior in terms of in vivo stability and mechanical strength thanks to the introduction of a water-soluble polymer as a linker which leads to an improvement in the reactivity of phenol or aniline moieties. Having the advantage of superior bio stability and mechanical strength, the hydrogel finds a variety of applications in the biomedical field.