Tunable　Diode　Laser　Absorption　Spectroscopy　(TDLAS)　applies　optical　method　to　realize　fast　and　non-contact　detection.　This　technology　has　the　advantages　of　high　resolution　and　high　sensibility,　and　been　widely　employed　in　atmosphere　environment　detection　in　recent　years.　This　paper　mainly　introduces　how　TDLAS　is　applied　to　detect　the　vehicle　exhaust,　to　determine　the　content　of　CO,　CO2,　NO　and　alkanes　etc.　Firstly,　the　principle　of　TDLAS　is　Lambert-Beer＇s　law.　It　is　based　on　the　specific　spectral　＂fingerprint＂　characteristics　of　different　gases　to　detect　the　gas　composition.　In　the　same　time,　the　light　intensity　is　attenuated　by　molecular　absorption,　which　is　used　to　accurately　analyze　the　density　of　gas.　There　are　two　ways　to　achieve.　One　is　direct-absorption　way,　whose　configuration　and　signal　processing　are　simple,　results　have　no　need　to　be　demarcated.　The　other　is　wavelength-modulation　way,　which　has　high　resolution　and　low　threshold.　The　selection　of　absorption　spectral　line　needs　not　only　to　adapt　the　central　wavelength　of　laser　and　responding　wavelength　of　detector,　but　also　to　avoid　the　crossed　spectral　line　of　different　gas　to　improve　the　precision　of　detection.　Secondly,　TDLAS　system　includes　three　parts,　which　are　signal　generation,　signal　detection　and　signal　processing.　Finally,　TDLAS　is　compared　with　other　spectral　detection　technologies　in　the　aspect　of　sensitivity,　character　of　working,　applicable　kinds　of　gas,　in　order　to　highlight　the　application　scope　of　each　technology.