The tunable diode laser absorption spectroscopy (TDLAS) technology and oxygen absorption lines near 760 nm are used to realize real-time on-line measurement of oxygen concentration. A detection system based on TDLAS direct absorption technique is designed. In order to solve signal amplitude fluctuation for on- site measurement, an automatic gain control module is designed using voltage controlled amplifier to achieve precise control of the spectrum signal amplitude; generalized Lorentz function is adopted to realize the approximate calculation of Voigt function combining Levenberg-Marquardt nonlinear fitting method to realize fast Voigt profile fitting of the absorbance spectrum curve, to meet the requirement of real-time on-line detection. Experimental results show that the algorithm can achieve Voigt profile fitting of absorbance curve. A fixed concentration of oxygen is continuously measured. Minimum detection limit of the system is 523 × 10^-6 m, and standard deviation is 1.75%. Detection system is reliable and meets the real-time on-line oxygen concentration detection applications.