In order to detect weak absorption signal overwhelmed in noise, phase-sensitive detection technique used in wavelength modulation spectroscopy system is studied, and a design scheme for digital phase-sensitive detection is presented. An algorithm of orthogonal vector structure is analyzed, and then the extraction of harmonic component is realized by hardware circuit design and computer software programming. The experiment is carried out for the measurement of O2 in open air by scanning two absorption lines of O2 near 764 nm, and the second-harmonic signal is successfully extracted. The result shows that the detection limit for volume fraction of the system is 0.50% (with the optical path of 1 m).