Infrared passive remote sensing allows remote monitoring and early warning of air pollution. The key to the quantitative resolution is to obtain the background radiation in application process. The existing methods (such as measuring the background and atmosphere radiance spectrum in the same condition or uptake) need in advance or at the same time to measure the background spectrum without target. So, they are difficult to satisfy the demand for rapid response in the practical application. According to the infrared radiation transmission model analysis and by studying the principle of measured spectrum in the 800-1200 cm-1 band containing the target, background and environmental radiation, we propose a real-time spectrum extraction algorithm that does not require to measure the background target feature. The method is applied to the developed polluting gas scanning imaging passive FTIR system. The remote sensing of SF6 is done. Concentration data and the data obtained by the method at the same condition are compared, and the results show that the correlation coefficient squared value reaches 0.99, indicating that the method is feasible, which can effectively improve the response speed of the system and the scope of application.