Four ionic liquids [BMIM]OH, [BMIM]IM, [BMIM]Br, and [BMIM]PF6 were synthesized and characterized by infrared spectroscopy. Then the effects of ionic liquids (ILs), cocatalysts, and reaction temperature on the catalytic performance for transesterification of ethylene carbonate and methanol were investigated with orthogonal experiments. The influence of cations and anions of ILs on catalytic activity was revealed by the density functional theory (DFT). The reaction mechanism was proposed based on the experimental results and DFT. The results demonstrated that the optimal catalyst was [Bmim]PF6/CaO, which exhibited the advantages of high activity, excellent stability, and easy recycling. Under the optimized conditions covering a catalytic temperature of 130 °C, an ionic liquid/cocatalyst mass ratio of 5:1, and a catalyst dosage of 4.0%, the conversion rate could reach 65.23% with a dimethyl carbonate selectivity of 98.95%. No significant loss of catalyst activity was detected after 7 recycle times.