The invention discloses a preparation method of rosin sizing agent and belongs to the technical field of deep processing of rosin. The preparation method includes: using the following materials according to parts by weight: 100 parts of rosin, 15-20 parts of soybean oil fatty acid, 5-8 parts of phosphoryl chloride, 1-2 parts of sodium acetate, 6-10 parts of polyethylene glycol, 0.1-0.2 part of sulfonated lignin, 15-25 parts of dimethyl silicone oil, and 10-15 parts of deionized water less than 5 MuS/cm in electric conductivity, wherein the method includes: heating and dissolving the rosin, adding the deionized water, and regulating pH to 9; adding the soybean oil fatty acid, the phosphoryl chloride, the sodium acetate, the polyethylene glycol, the sulfonated lignin and the dimethyl silicone oil, heating to 80-95 DEG C for reacting for 0.5 h, cooling to 55-65 DEG C for 1 h, and heating to 80-95 DEG C for reacting for 0.5 h. The rosin sizing agent prepared herein has high stability.