Polyols with different number of hydroxyl groups were synthesized from ricinoleic acid and oleic acid. These plant oil derived polyols can be used as plasticizers and combined with ethyl cellulose (EC) through supramolecular system to prepare ethyl cellulose composite films. The morphology, thermal stability and mechanical properties of the prepared ethyl cellulose films were studied in detail to indicate the plasticizing efficiency of these polyols and the effect of the number of hydroxyl groups in polyols on EC composite film’s properties. With the addition of polyols, the tensile strength of EC composite films decreased, while the elongation at break increased. The elongation at break of EC composite films can be improved by 11–12 times when the polyols contain 1–3 hydroxyl groups. Thus, the plant oil derived polyols with 1–3 hydroxyl groups can be used as plasticizers for EC films. In addition, molecular dynamic simulations were performed to further probe the effects of the number of hydroxyl groups in polyols on EC composite film’s properties. Graphic abstract