Abstract: After the Fukushima nuclear accident, the safety of spent fuel pool has attracted much attention. Under extreme accident conditions, the spent fuel pool may lose make-up water and cooling for a long time, causing the water level in the spent fuel pool to drop, resulting in excessive temperature of spent fuel rods, even damage to fuel assemblies and leakage of radioactive substances, come into being serious safety problems. In order to avoid this safety problem, the third generation nuclear power plants, such as AP1000 and CAP1400, introduced spray cooling system. Under the condition of spraying, the liquid film flow characteristics on spent fuel rods is an important factor affecting the cooling effect, which has not been studied in detail by domestic and foreign scholars. Too large or too small spray flow rate may lead to the rupture of the liquid film on the spent fuel rod. Therefore, it is necessary to study the flow characteristics of the liquid film on the spent fuel rod. In this paper, optical method was used to study the time and space variation of liquid film thickness formed by spray cooling of single spent fuel rod at different Reynolds numbers. The liquid film image was captured by CCD camera, and the clear liquid film thickness image and good coincidence data were obtained after processing. Using the obtained data, the fluctuation images of liquid film at different Reynolds numbers at fixed positions of spent fuel rod and at different positions of spent fuel rod at fixed Reynolds numbers were drawn. In addition, the variation trend of time-averaged liquid film thickness on spent fuel rod at different Reynolds numbers was obtained. The experimental results show that when Reynolds number is in the range of 608-7 538, the maximum value of transient liquid film thickness is 2.36 mm, which occurs when Reynolds number is 7 085. With the increase of Reynolds number, the time mean liquid film thickness will increase, and the amplitude of liquid film fluctuation will also increase. Along the rod direction, with the increase of the distance from the top of the rod, the liquid film thickness will gradually decrease and become stable, and as the Reynolds number increases, the stable part will appear farther from the top of the rod. The research on the flow characteristics of single rod coolant film of spent fuel lays a foundation for determining the minimum spray flow rate with effective cooling capacity.