According to the relation,which between species concentration and modulated depth of resonant susceptibility to non resonant susceptibility in low concentration,a method is presented to measure the low concentration species in other species.A formula is given between modulated depth of resonant susceptibility,gas concentration and temperature,which its correction is verified by experiment.The CO concentration of different position in alcohol frame is measured.The error between theoretical value and experimental value is lowered to 4.5 %.Further,Influence of temperature to testing result is discussed.