Abstract: Spur dikes lead to a change in the flow structure and have an impact on the local scour around its head and the evolution of riverbed nearby. River width narrowing ratio (i.e. the ratio of spur dike length to river width) is a key factor of its local scour features. Based on numerical simulations of the 3D flows, this paper analyzes the scour depth and riverbed deformation around the head of a non-submerged spur dike at different narrowing ratios. Results reveal that the maximum scour depth essentially linearly increases with the narrowing ratio in its range of 0.5 or less but it increases more rapidly at the ratio beyond this range. In the latter case, the scope of a spur dike on the flow reaches the opposite bank, and consequently the scour pit also extends to the shore, taking a shape greatly different from those at the smaller ratios. This indicates the significance in selecting a suitable river width narrowing ratio in design of practical spur dikes