In order to measure flue gas velocity and particulate concentration by optical non-intrusive method, a system is developed with double light paths structure, and is tested on industrial stack. The transmitting and collecting units locate at bilateral of the stack. The gas flow in stack is very complex and changes quickly, thus an inverse cascade model is proposed to describe the developing process of gas flow, and the data processing method for calculating flue gas velocity is improved. Analytical results indicate that, the difference of statistical mean value of flue gas velocities calculated by the new method and those measured by pitot tube at a single point is 0.7 m/s, which is larger than the expectation value, but the ensemble average velocities obtained by these two kinds of measurement are almost the same and are all about 7.6 m/s. When signals collected by our optical measuring system are perfect, the new method will not change the value of velocity and its trend is explicitly acquired after simple filter. There is clearly linear relationship between light intensity fluctuation measured by flue gas velocity measuring system and particulate concentration in stack measured by optical particle counter, and the correlation coefficient can reach 0.97.