A modified wavelength modulation spectroscopy (WMS) based on the self-heating effect of the tunable diode laser when driven in quasi-continuous-wave (QCW) mode is investigated. A near-infrared distributed feedback (DFB) diode laser working at the QCW mode is employed as the QCW light source, and CO2 is selected as the target gas. The characteristic of the QCW second harmonic (2f) line profile is analyzed through a comparison with that of the traditional CW WMS with the same system. A noise-equivalent absorbance of 3.2×10 5 Hz 1/2 for CO2 at 1.58 \mu m is obtained with 18-m optical path. The QCW WMS lowers the dependence on lasers and expands selectivity, thus verifying the feasibility of the method.