A novel measurement system for the gas density based on spectrum absorption is studied in details. Based on the theory of Beer-Lambert, the technology of nonlinear laser diode (LD) spectrum is used to perform the polluting gases monitoring on line. The peak wavelengths of LD are changed with the applied voltage and temperature, so the technology of wavelength modulating or frequency modulating can be used at perform the measurement in wide scope. In this paper, the low density of the polluting gases, is detected by the method of harmonic detection in order to increase the detection sensitivity.