The mid-IR femtosecond sources based on difference-frequency generation exhibit many attractive features such as wide wavelength tuning range (6-20 μm), wide coverage range (the entire “fingerprint” region), and low system complexity. Ultrafast fiber laser driven mid-IR fs sources have higher system stability for using less spatial light path only in difference-frequency part. In this article, the different fiber nonlinear frequency conversion techniques in difference-frequency generation were reviewed. The methods improving mid-IR pulse power in different-frequency generation process were introduced in detail.