The mid-infrared spectrum region, which corresponds to the fundamental absorption of molecules, can be applied to high-sensitivity trace gas measurement. A method using a mid-infrared continuous-wave quantum cascade laser (CW-QCL) operated in room temperature with wavelength modulation spectroscope (WMS) is demonstrated. A method of eliminating influence of cross interference between different gases is studied and related verification experiment is performed. An open path greenhouse gas detection system using a quantum cascade laser with central wavelength around 1274 cm-1 is constructed and utilized to monitor CH4 and N2O in atmsphere simultaneously with a 101 m open optical path measurement test, which achieves detection limits of 3.87×10-9 and 1.28×10-9 respectively. The feasibility of the experiment system and the method is verified, which lays a foundation for high-sensitivity detection of greenhouse gases in regional environment.