The optimization of electron beam parameters and the design of the optical cavity have been numerically carried out in order to obtain high FEL gains in the KU-FEL facility. A FEL gain of up to 82 % has been reached for the FEL wavelength of 12 μm in the optimized conditions. The FEL power saturation in 8∼12 μm wavelength can be expected with the optimized condition. However, it is still difficult to obtain in the FEL saturation below 6 μm wavelength with a macro-pulse duration of 3 μs or less. In addition, the calculated FEL gain increases with the normalized filling factor. Consequently, increasing the density of electron beam and the overlap between the electron beam and the laser is important to obtain a large FEL gain.