Abstract: Using ultrasonic wave and PIV technical methods for measuring water level and velocity respectively, the influences of different hydraulic gradients on the flow structures near a submerged spur dike are studied by laboratory experiments in a U-shape flume. Experimental results show that under the same hydraulic gradients, the transverse slope of water surface along the longitudinal direction increases in the upstream of the dike, but it decreases in the downstream of the dike. Under different hydraulic gradients, with the increase of hydraulic gradients, it is found that: ① the water level, transverse slope of water surface, relative flooding depth in the vicinity of the dike and the water level difference between the upstream face and downstream face of the dike will gradually decrease; ② the position with the highest water level in the upstream of the dike moves downward, while the water level recovery point in the downstream moves upward; ③ the transverse mean flow velocity and the maximum flow velocity gradually increase, but the position of the maximum flow velocity almost does not change; ④ the vortex velocity distribution is relatively similar with an increase of vortex intensity, while the center of the main vortex behind the submerged spur dike moves toward the opposite bank of the downstream. When the flume slope changes from -1‰ to 1‰, the submerged spur dike has a little effect on the flow patterns in the area of 5 cm above the dike. However, when the flume slope reaches 2‰, the submerged spur dike has an obvious effect on the flow patterns in the same area. It implies that there may exist a critical value between 1‰ and 2‰ for the flume slope that affects the submerged spur dike and flow patterns. The research results will be favourable to the deep exploration of the complicated environmental hydraulic problems in the fields of fluvial processes and coastal engineering.