For the proven technology and fast noncontact advantages of gas concentration measurement, the tunable diode laser absorption spectroscopy (TDLAS) is very suitable for the residual oxygen concentration detection in the packaged Xilin bottle. In this paper, TDLAS is used for residual oxygen detection in the packaged xilin bottle. The detection light goes through the air and glass bottle, and the scattering and attenuation to the glass wall of the laser are the main interferences which have a great impact on the stability of the second harmonic signal. This paper designed and built the residual oxygen detection system in packaged bottle based on TDLAS. According to the second harmonic signal collected from the system, a new method based on wavelet transform is proposed and the effect is obvious. Firstly, using “sym6” wavelet, the measured signal was decomposed by five layers of wavelet, and the corresponding baseline slope was obtained according to the low frequency component of each layer. Then, the baseline slope of the original signal was obtained by weighted average of the five baseline slopes. According to the obtained baseline slope, the original signal was processed by the baseline, and the reconstructed signal was obtained after the wavelet decomposition and the soft threshold processing. The measurement results of the bottle with 21% oxygen concentration showed that the relative error between the treated and the theoretical signals decreased from 1.26% to 0.12%, which proved that this method could solve the problems of baseline drift and noise interference in the process of residual oxygen concentration detection, and overcome the interference of the glass wall to the second harmonic signal, providing a high quality signal for measuring oxygen concentration.