Abstract: Step-shaped spur dike is widely used in the waterway regulation, and the law governing the backflow is closely related with the problems such as ship navigation, siltation of dam field(deposition between two groins) and embankment stability, but few studies are conducted in this field. In this research, the influence of spur's geometric scale and inflow condition on the backflow is studied by using a three dimensional mathematical model of water flow. The results show that: ①The backflow region length increases with the increasing of the relative length ε₁ and relative height ψ₁ of the first stage spur dike and the relative length ε₂ of the second stage spur dike, and it increases first and then decreases with the Froude number Fr; ②The backflow region width changes relatively obviously with ε₁, ε₂ and ψ₁, and changes little with Fr overall; ③The width ratio curve of the positive velocity region and the negative velocity region is overall convex, and the negative flow width is relatively large, which can be 2.5 times the width of the positive velocity region near the spur dike; ④The shape of backflow curve is related to ε₁, ε₂ and ψ₁, and ε₁ and ψ₁ mainly affect the length, while ε₂ affects the overall shape; ⑤The circumfluence flow is less than 10% of the discharge, and its distribution curve is asymmetric, with slow change in the flow increasing area and relatively severe change in the flow decreasing area.