A novel solid acidic ionic liquid polymer (PIL) has been synthesized through the copolymerization of acidic ionic liquid oligomers with divinylbenzene (DVB). An efficient procedure for biodiesel synthesis from waste oils with a high free fatty acid (FFA) content was developed. PIL could efficiently catalyze both the transesterification of triglycerides and the esterification of FFAs with a total yield of >99.0%. The catalytic activities were quite high, with just one step to complete the reactions simultaneously under very mild conditions. The high activities are due to the high hydrophobic surface area, high acidity, and high stability of PIL.