A long open path fourier transform infrared spectroscopy system is described. The 250-meter-optical-path configuration is combined with a Fourier transform spectrometer of 1 cm(-1) resolution. With an active blackbody source, the transmittance spectrum of the atmosphere could be measured by the system. And then, it was fitted with the nonlinear least square algorithm to calculate the average concentrations of the constituents in the path. Measuring result of some gases such as CH4, CO, N2O and CO2 in polluted air was presented in the experiment part and the RMS of the residuals was <1%. The result shows that the system is stable and rapid, and can be easily aligned. It is suitable for research work on some infrared active important atmospheric constituents.