The paper is concerned with the method of CO2-laser spectrum control based on rotovibrational lines competition and laser signature dependence on cavity length. The method gives an advantage of the more full utilization of active medium energetic potentialities. A laser signature stability range have been studied experimentally and theoretically, and both results agree well. Design of the LUS type CO2-laser (LUS is Russian abbreviation for Spectrum Controlled Laser) and LUS applications in atmospheric differential absorption spectrometry are briefly reported.