Abstract: Flow characteristics of the coolant inside the nuclear fuel rod bundle is an important research topic for reactor thermal-hydraulic safety analysis. Visualization experiment results of the rod bundle with spacer grid were studied in this paper, and fluid mixing inside the rod bundle was analyzed qualitatively and quantitatively based on image processing algorithm, to provide the information for the reactor thermal-hydraulic safety analysis. Image processing algorithm was introduced at three levels, i.e., data verification, regularity summarizing and mechanism analysis, and the image processing algorithm was applied in analyzing of visualization experiment results of rod bundle. The analysis results indicate that the flow mixing mitigate 5 cm downstream the spacer grid, and Canny edge detector is applicable of spacer grid turbulence image.