Abstract: In order to study the effect of phase difference between frequency modulation and amplitude modulation on both the second harmonic signal and its line shape,the mathematical derivations for wavelength modulation spectroscopy were presented in consideration of residual amplitude modulation,and then the theoretical calculation formulas of considering FM-AM phase difference were deduced.In addition,the impact of FM-AM phase difference on harmonic signal line shape was also simulated and calculated.The results show that the second harmonic signal positive peaks and the left and right negative peak as well as the signal baseline are all changed with the FM-AM phase difference.However,the distance between left and right negative peak value is not affected by it,remaining unchanged.The calculation results have important significant on tunable diode laser absorption spectroscopy gas detection.