Aiming at the narrow spectral range and wide pulse width of domestic mid-infrared laser sources, research on mid-infrared femtosecond laser technology based on difference frequency generation (DFG) was carried out. The mid-infrared femtosecond laser generated based on DFG has the advantages of a wide spectral range and narrow pulse width. The influence of different nonlinear crystals on the generated mid-infrared laser in DFG was studied. On this basis, a mid-infrared laser generation system based on DFG using a PPLN crystal was conducted. Further, this system used grating pairs to compress the pulse width to achieve femtosecond laser output. Finally, a mid-infrared femtosecond laser with a tunable wavelength range of 2.9 ~ 4.7 μm was observed, and the highest average output light power of 10.46 mW was obtained at a central wavelength of 3.2 μm. The research results provide a reference for the application of mid-infrared laser spectroscopy measurement technology in atmospheric monitoring, combustion field component detection, etc.