The invention relates to the technical field of chemical adhesives and aims at providing a preparation method of a dual-component solvent-free type polyurethane adhesive for composite flexible packaging. The preparation method comprises the following steps: (1) preparing an A component: sequentially adding polyether polyol with -OH end, castor oil, natural polysaccharides and a catalyst into a reaction kettle, stirring, heating to 60-80 DEG C, and performing vacuum dehydration; and cooling to room temperature, discharging, sealing and packaging; (2) preparing a B component: adding castor oil and polyisocyanate into the reaction kettle, stirring, and performing vacuum dehydration; adding stannous octoate and triethylene diamine under nitrogen production conditions, and heating to 60-80 DEG C; stopping the reaction when the content of NCO is constant, cooling to room temperature, discharging, sealing and packaging; and mixing the two components according to an OH / NCO functional group. According to the preparation method disclosed by the invention, an organic solvent is not used, the pollution is avoided, and the safety is high; raw materials are green, environment-friendly, degradable, easy to obtain and low in cost; the dual-component solvent-free type polyurethane adhesive further has the advantages of water (hydrolysis) resistance, flexibility and low temperature resistance; and a natural polysaccharide is used as a cross-linking agent, so that the cross-linking density and the bonding strength are improved.