Abstract: To study the height effect of the pebble flow dynamics, a 2-D pebble-bed experimental system for High Temperature Gas-cooled Reactor（HTGR） has been established based on the similarity criteria to study the pebble flow by using the marking pebble method. The characteristics of mean stream line, inner flow distribution, mean stagnant time, and distribution of the pebbles in the mixing zone of the 1m and 2m experimental system have been discussed and compared. The results show that the mean streamline is smooth and symmetrical and the flow characteristics in the side area and the central region are greatly different. The pebble flow in the lower zone of the experiment set is more nonuniform than that in the upper zone. The degree of dispersion can be enlarged by increasing the height of the bed. These results validate the Gaussian Probability Distribution of pebble in the mixing zone. The moving area of the marking pebble has a wide middle section and narrow head and end sections. The degree of dispersion is larger in the higher bed than that in the lower bed.