Abstract: Since the first use of Kerr lens mode-locked Ti: sapphire laser pumped RTA optical parametric oscillator to achieve mid-infrared femtosecond laser in 1994, with the continuous emergence of high-power near-infrared pump sources and various high-quality nonlinear crystals, the mid-infrared femtosecond optical parametric oscillator had made considerable progress in terms of average power, pulse width, and tuning range in the past two decades, providing diverse application tools for basic scientific research, biomedicine, and national defense security. Mid-infrared femtosecond optical parametric oscillators were divided into two types in this paper: wavelength tunable output type and broadband-spectrum output type. The research progress of these two types of 2-5 µm mid-infrared femtosecond optical parametric oscillators at home and abroad were reviewed respectively. Finally, the further development trend was discussed. In view of the outlook, high-power, high-beam quality mid-infrared femtosecond optical parametric oscillators and high-energy mid-infrared femtosecond optical parametric oscillators are two important development directions.