An in situ formed hydrogel was synthesized by sodium alginate and cysteine methyl ester, which turned the sodium alginate into thiolated alginate (SA-SH). SA-SH can in situ formed into hydrogel (SA-SS-SA) with a large amount of water through covalent bond in less than 20 s. The structure characterization showed that the mechanism of SA-SH gelation was thiol-disulfide transformation. The rheology and cytotoxicity experiments of SA-SS-SA hydrogel were also investigated, which indicated that SA-SS-SA hydrogel had an appropriate mechanical strength as well as an excellent biocompatibility. The SA-SS-SA hydrogel would degrade under certain conditions after a few days and its mechanism was disulfide alkaline reduction. Finally, the hemostatic property of SA-SH was tested by rat tail amputation experiment. The time to hemostasis of rat reduced from 8.26 min to 3.24 min, which proved that SA-SH had an excellent hemostatic property.