Sulfur dioxide (SO2) and sulfur trioxide (SO3) are important substances in waste gas emission, and do great harm to both the environment and human's health. However, in-situ and real-time measurement of SO2 and SO3 in the process of emissions has always been a challenge. Herein, tunable diode laser absorption spectroscopy (TDLAS) technology is presented to determine the mass of SO2 and SO3 simultaneously using a quantum cascade laser emitting at 7.16 μm, and the sensitivity and robustness of the measurement system is improved by employing wavelength modulation spectroscopy. In this work, a simple single-pass gas cell with low-volume is used at high temperature and low pressure to ensure the spectra of SO2 and SO3 can be separated completely when measured simultaneously. Therefore, the concentration measurement of both SO2 and SO3 are reliable, since their spectral lines are not affected byspectral overlapping due to broadening. In addition, the influence of temperature on SO2 concentration measurement is corrected, and it is proposed to calibrate the concentration of SO3 with the standard concentration SO2 gas. According tothe Allan deviation analysis, the minimum detection limit for SO2 can reach 1.98×10-6 cm3·cm-3 with the integration time of 34 s, and that of SO3 is estimated as 1.575×10-6 cm3·cm-3. The rise time and the fall time of the system are rapid as 16 s and 18 s, respectively.