The invention relates to a preparation method of a castor oil based polyurethane-acrylonitrile composite emulsion adhesive for leather. The preparation method comprises the following steps: mixing 10-20 wt% of castor oil, 30-40 wt% of polytetrahydrofuran glycol, 10-30 wt% of diisocyanate and 0-5 wt% of a catalyst, reacting for 1-6h in the presence of N2 and at a temperature of 50-100 DEG C, adding 4-8 wt% of bis (hydroxymethyl) tallate, reacting for 1-6h at a temperature of 50-100 DEG C, neutralizing the obtained object by using 3-6 wt% of triethylamine, adding an enough amount of deionized water, stirring the obtained mixture at a speed of 3000 r/min so as to obtain a polyurethane emulsion with a solid content of 40%, adding 10-20 wt% of acrylonitrile and 1-6 wt% of an initiator into the obtained polyurethane emulsion, and reacting for 1-6h at a temperature of 50-100 DEG C, so that the castor oil based polyurethane-acrylonitrile composite emulsion adhesive is obtained. The preparation method disclosed by the invention is an effective method for preparing environmental-friendly emulsion adhesives by using renewable resources.