As a new advanced method of measuring trace gases,the technology of tunable diode laser absorption spectroscopy(TDLAS)has been applied in the fields of atmospheric chemistry and pollutant gases monitoring. When the absorption optic length can not be effective increased in the system,the noise of system will limit the capabilities of the system and should be reduced to increase the detection limit of TDLAS system. The origin of noise of TDLAS system is introduced and the second-harmonic detection of CO and CO₂ with shortpath optic length at atmosphere pressure in the near-infrared wavelength also presented. The detection limit of CO is 0.73% and the one of CO₂ is 0.98%. It is obvious that the TDLAS system is fit for the environmental monitor with the high detection limit.