To monitor the velocity of industrial exhaust gas continuously, a parallel double-path flow velocity measurement system was designed, and the related theories on measuring the velocity of stack gas flow was studied based on the light scintillation. A simple phase-screen technique was used to analyze the light scintillation in a turbulent medium, and the expression of optical scintillation cross-correlation was obtained which is used to measure stack gas flow velocity. The path-weighting function of the mean velocity was given. The numerical simulation carried on path-weighting function indicates that it has axially symmetric distribution as Gaussian line shape, and the central velocity of the stack makes greater contribution. The impact of light source spectrum on path-weighting in principle was analyzed, and the preliminary experiments show that, although because the light source spectrum has a certain bandwidth, the path-weighting function has changed, the distribution is still axially symmetric. Based on the path-weighting function, the space distribution of the gas flow velocity in stack can be calculated.