Abstract: Large-scale artificial roughness (LSAR) is a common element of bed roughness in mountain rivers. To investigate the influence of LSAR morphology on surrounding flow structures, three LSAR shapes (cube, sphere, and tetrahedron) with the same height (Δ) were used in a single flume experiment. Instantaneous flow velocity data were acquired using an Acoustic Doppler Velocimeter. The results show that: (1) When the water depth is small (h/Δ=0.9) and the LSAR is in a non-submerged state, the three LSAR shapes are surrounded by a downward U-shaped flow. The transverse flow area around the cube is the largest, the second largest is around the sphere, and the smallest around the tetrahedron. When the water depth increases (h/Δ=1.8) and the LSAR is in a submerged state, the transverse flow around it clearly weakens, and water flows over the LSAR and forms undulating downstream waves. (2) The resulting surface velocity U_m/U increases as the Froude number (Fr) and the water hindrance area increases. When Fr is large, a high velocity zone in the surface layer appears near the plane centroid of the LSAR, and the maximum velocity in this zone can be up to 1.3 times the test velocity. The distribution shape of U_m/U in the surface layer around the cube, sphere, and tetrahedron is that of an isosceles trapezium, bell-shaped, and rhombus, respectively. The high transverse velocity zone is in the 1.5Δ range on both sides of the plane centroid of the LSAR. The maximum value can be up to 0.19 times the test velocity. (3) When Fr is small, the LSAR has little influence on the vertical distribution of longitudinal velocity u/U. In the upstream section, u/U essentially follows a exponential distribution, and the distribution clearly changes at 1 Δ downstream of the LSAR. Subsequently, the vertical distribution of u/U gradually returns to an exponential distribution. As Fr increases, the influence of LSAR on the vertical distribution of u/U gradually increases, and the distribution of u/U is no longer exponential.