In the infrared gas detection system based on tunable diode laser absorption spectroscopy (TDLAS), the second harmonic signal is used to calculate the gas percentage by volume. However, this signal is easily influenced by many parameters of the hardware. Based on Lambert Beers law and HITRAN database, this paper used LabVIEW to study the influence of parameters on the amplitude, symmetry and peak width of the second harmonic, and carried out the harmonic optimization simulation experiment. The results show that, for the general TDLAS detection system, the appropriate scanning amplitude and filter order should be chosen firstly, so that the harmonic line would be complete and undistorted. Furthermore, the amplitude and peak width of the harmonic should be increased as much as possible to get the optimal modulation depth. Finally, the suitable modulation and scan frequency should be determined choose according to the system requirements and the hardware conditions.