Difference frequency generation is an important method to obtain 3~5 μm mid-infrared laser. It does not need complex structure adjustment of cavity, the tuning range of output laser is wide, and the efficiency is high. We derive the formulas of output wavelength, linewidth and output power of the mid-infrared laser obtained from difference frequency generation. The dependences of output linewidth on pump light wavelength, pump light linewidth, signal light linewidth and temperature are emphasized. Numerical results show that the output linewidth is mainly decided by the pump light linewidth and the signal light linewidth.