Disclosed is a medical hydrogel formed by in situ crosslinking of aldehyde-terminated multi-arm star polyethylene glycol and a polyamino compound. The aldehyde group and the multi-arm star polyethylene glycol are linked by a chemical bond, such as an ether bond, amide bond, urethane bond, imine bond, and urea bond. The present invention utilizes the reaction between the aldehyde group at the end of the multi-arm polyethylene glycol and the amino group of the polyamino compound to form a Schiff base, resulting in crosslinking, thereby forming a medical injectable gel. The resulting gel has a shorter gelation time and ideal gel bursting strength, is stable in an aqueous solution, and has higher application value than an existing medical gel.