The invention relates to medical injectable bonding gel and a preparation method thereof. According to the preparation method, the gel is formed in situ by mixing a properly modified dextran water solution and a multi-amino compound crosslinking agent solution. A mixed solution of the bonding gel is in a liquid state at the initial stage, seams in the wound surface can be quickly filled with the mixed solution along with a chemical crosslinking reaction, and the mixed solution is gelatinized, so that the gel which is immobile and has certain bonding strength can be formed finally. The gel causes no oppression on nerves, is free of influence of body position, and can bear pressure between tissues; the degradation time is relatively long, so that the wound part can completely keep durable bonding strength until the part is completely healed; the gel is nontoxic and non-irritant to in-vivo tissues, and thus can be degraded or absorbed in vivo finally; gel can be widely applied to postoperative bonding and sealing between body tissues as well as between implant materials and body tissues in departments of neurosurgery, brain surgery, general surgery, orthopedics, cardiothoracic surgery, urinary surgery, gynaecology and obstetrics, and the like.