Differential optical absorption spectroscopy (DOAS) has become a widely used method to measure trace gases in the atmosphere, which detects trace gases by analyzing the typical absorption waveband of narrow band molecules. The concentration of trace gases is retrieved through least-squares fits of reference absorbing cross section to the differential cross section. But in the application, for the superposition of noises on the absorption spectra, the measurement precision is affected. In the DOAS, denoising by polynomial smoothing filtering is the traditional method. Wavelet transform with soft threshold is put forward to reduce noises, which improves the precision of DOAS and declines the measuring limit, according to the comparison with experimental results.