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.