Based on the laser Spontaneous Raman Scattering (SRS) technique, a multi-channel optical diagnostic system is developed for quantitative measurement of mole fraction of gas. A laser pulse stretcher is designed for the 532 nm green laser to efficiently avoid any unwanted phenomena including gas cracking and quartz glass damage due to the impact of high-energy of pulsed laser. The system can also be used to enhance the signal-to-noise ratio of gas Raman scattering spectrum. Under a standard atmospheric temperature and pressure, Raman scattering measurements are carried out in an air excitation zone within a gas cell. The length and the diameter of the cylindrical excitation zone are 66 and 1 mm respectively. Raman spectroscopy and mole fraction of O2 and N2 within each channel are obtained from the tests, together with the relative response factor of O2 to N2, the RO2. In total, 26 repeated experiments are conducted; and all of them are deduced from an accumulated spectrum of 200 laser shots. It shows that the standard deviations between the results of each channel in terms of the averaged mole fractions of O2, O2 and the averaged relative response factor of O2/N2, O2 are 0.015 and 0.024, respectively. However, the channel-averaged value of these parameters is exactly the same as the result obtained with a merged-channel-mode. The accuracy of the measurement is 98%, indicating that this system can fully meet the requirements of real time quantitative measurement for mole fraction of gas mixture with providing spatial and temporal resolution. The present system satisfies the spectral measurement and analysis of various dynamic combustion processes.