The invention discloses a modified-starch-based wood adhesive and a preparation method thereof. The preparation method comprises the steps that oxidized corn starch is used as a main raw material, a cross-linking agent is added into the oxidized corn starch to conduct crosslinking modification, and then an initiating agent and a high molecular monomer are added to the oxidized corn starch to conduct graft modification, so as to obtain a modified starch emulsion in a complex net structure; and after the modified starch emulsion is warmed up and pasted, modifying agents comprising a stabilizing agent, a defoaming agent, a plasticizing agent and the like are added into the modified starch emulsion, and the high-performance green environment-friendly adhesive is manufactured. Each performance of the adhesive disclosed by the invention can reach or exceed requirements of a GB/T9846-2004 standard, the adhesive has the characteristics of high bonding strength, good water resistance, long storage time, high solid content, low cost, greenness, environment friendliness and wide use range, and can be widely applied to the bonding and the use of wood materials such as artificial boards, plywood and cellulose boards.