Since the light intensity is modulated in the wavelength modulation spectroscopy (WMS) in the trace gas detection based on tunable diode laser absorption spectroscopy (TDLAS), the output signal can be accompanied by the residual amplitude modulation (RAM), which can affect the line shape of the detection signal and the noise to a certain extent. Theoretical model and analysis of the second harmonic signal of arbitrary large modulation amplitude based on Fourier analysis are presented, and the theoretical explanations about the distortion of the absorption lines are given. The effect of linear intensity modulation and non-linear modulation on the spectral distortion is analyzed, and the basic reasons of the asymmetry of the second harmonic and the noise are given. Considering the intensity amplitude modulation, the experiment of the the second harmonic detection of NH3 based on the TDLAS is conducted and the effect of the modulation amplitude on the distortion of absorption lines is evaluated. The experimental results confirm the theoretical analysis.