The synthesis of rosin acid starch ester catalyzed by lipase under the micro-solvent system was studied by using the materials of rosin and cassava starch and tertiary amyl alcohol as the solvent. The effect of the factors, including rosin-starch ratio, reaction time, reaction temperature and dosage of lipase, on the degree of substitution (Ds) of rosin acid starch ester were mainly analyzed. The optimal condition was rosin-starch ratio 4∶1, reaction temperature 55℃, reaction time 10h,dosage of lipase 4%. The structure of the product was analyzed by FT-IR, XRD and SEM. The results showed that a C=O stretching vibration absorption peak at 1725cm-1 was observed, which indicated the rosin and starch was formed into rosin acid starch ester, with keeping the original crystal morphology and structure of starch. And the lipophilicity of rosin acid starch ester was investigated. Compared with the cassava starch, the oil-absorption capacity of rosin acid starch ester for olive oil was increased by 166.67% in octane. The reaction of micro-solvent system largely reduced the cost and increased the quantity of products.