In the present paper, the signal of a tunable diode laser absorption spectroscopy (TDLAS) trace gas sensing system, which has a wavelength modulation with a wide range of modulation amplitudes, is studied based on Fourier analysis method.Theory explanation of spectrum distortion induced by laser intensity amplitude modulation is given.In order to rectify the spectrum distortion, a method of synchronous amplitude modulation suppression by a variable optical attenuator is proposed.To validate the method, an experimental setup is designed.Absorption spectrum measurement experiments on CO2 gas were carried out.The results show that the residual laser intensity modulation amplitude of the experimental system is reduced to ~0.1% of its original value and the spectrum distortion improvement is 92% with the synchronous amplitude modulation suppression.The modulation amplitude of laser intensity can be effectively reduced and the spectrum distortion can be well corrected by using the given correction method and system.By using a variable optical attenuator in the TDLAS (tunable diode laser absorption spectroscopy) system, the dynamic range requirements of photoelectric detector, digital to analog converter, filters and other aspects of the TDLAS system are reduced.This spectrum distortion correction method can be used for online trace gas analyzing in process industry.