Based on the mid-infrared quantum cascade laser (QCL) and wavelength-modulation frequency-division multiplexing spectroscopic technique, the simultaneous and high-sensitive NO2 and NH3 concentration measurements are realized. The QCLs with central frequencies around 1600.0 cm -1 and 1103.4 cm -1 are modulated at different frequencies. With the digital lock-in technique, the second harmonic signals are obtained by demodulating the original signals at different frequencies. A measurement system with a 60 m open-path multi-pass cell is developed for simultaneous monitoring of NO2 and NH3 in the air. A 25-cm-long reference gas cell is used to calibrate the system and it is found that the excellent linear response of the system occurs within a wide range of concentration. The detection limits for two kinds of gases are smaller than 10 -9. The real-time gas monitoring during 24 h is performed with this system and the measurement results show a good agreement with those given by the reference instruments.