We theoretically study the nonlinear compression of the 10-mJ, 62-fs, 3.6-µm laser pulses in an argon gas-filled hollow-core fiber with large diameter of 1000 µm. Using a pressure gradient to restrict undesirable nonlinear effect such as ionization, especially at the entrance, we obtain the intense 18.3-fs (~1.5 cycle) pulses at 3.6 µm only through compression with CaF2 crystal, which can be used as an ultrafast source for strong field driven experiments. In addition, we calculate and discuss the relation between optimal fiber length and coupling efficiency for a given bandwidth. These results are useful for the design of using hollow-core fiber to compress the high-energy pulses with long wavelength.