High sensitivity for gas detection can be achieved by modulation spectroscopy technology based on harmonic detection, and the best analytical performance depends on the system modulation parameters optimization. Harmonic signal characteristics of infrared laser modulation spectroscopy is studied, and the relationship between the modulation parameters and corresponding harmonic signals is analyzed. Experiments and validation analyses are carried out for the typical infrared diode laser spectroscopy gas detection system, the optimal modulation system parameter is determined through the experimental analysis of the harmonic signal characteristics for different modulation voltages.