We experimentally demonstrate a high-speed mid-infrared (MIR) transmission system with a 5 Gbaud quadrature phase shift keying (QPSK) modulation over free space optical (FSO) link. The MIR system operates through homemade robust and compact MIR transmitter and receiver devices, which has the ability to achieve wavelength conversion between C-band and MIR based on difference frequency generation (DFG) process. The wavelength and power of MIR signal generated from MIR transmitter are 3.57 dBm and 3594 nm, and the C-band signal regenerated from MIR receiver are −26.12 dBm and 1550.116 nm. Both the constellation diagrams and eye diagrams of regenerated C-band signal are clear when the input power of C-band receiver is higher than −40 dBm. Compared with the back to back (BTB) transmission system, the power penalty of the MIR transmission system is less than 2.8 dB measured at bit error ratio (BER) of 1×10−7. It can be concluded from the experimental results that the MIR transmission system supports high-speed data rate, variable wavelength and high-order modulation format, and has the potential to be applied to FSO communications or satellite communications in atmospheric environments. © 2020 Elsevier B.V.