This invention discloses a preparation method for biocompatible rapid-gelating hydrogel. Wherein, hydrogel is formed by rapid chemical-crosslinking using the mixing and chemical-crosslinking reaction under specified conditions between several active compound components. The preparation method comprises the following steps: (1) The solution containing biocompatible thiolated macromolecular derivatives (component A) and biocompatible thiol reactive crosslinking agents (component B) mutually mix to form reactive mixture with specified crosslinking conditions; (2) the reactive mixture forms the hydrogel. The invention also discloses a preparation method for novel rapid-gelating hydrogel spray and an application in medical field. This invention has the advantages of good biocompatibility, no by-products, good stability, convenient use, small amount of raw materials used, suitable for many medical application, etc.