Direct hydrogenation of CO2 into formic acid is an atom economic reaction, but it is thermodynamically limited, and the production of free formic acid is challenging. Herein, we report a protocol of CO2 hydrogenation to free formic acid over Pd/C in ionic liquid (e.g., 1-butyl-3-methylimidazolium acetate, [Bmim][OAc]) without base under mild conditions. Detailed study indicates that [Bmim][OAc] plays multiple roles in the reaction process with activating CO2, modifying the Pd nanoparticles to improve their activities, and stabilizing generated formic acid. This Pd/C-[Bmim][OAc] catalytic system is very effective for the production of free formic acid at 40 degrees C, affording a production rate of 233.5 mmol-g(-1).h(-1) and turnover number (TON) of 594, and the concentration of free formic acid in the reaction solution could reach up to 5.20 mmol/L.