Abstract: The impact of granular flow such as debris flow and landslide, and how to design obstacles to deflect granular hazard, are becoming more and more important recently. In this study a bed-fitted depth-averaged model is established to simulate the interaction between granular flow and obstacles on steep terrains, which is able to simulate the birth and evolution of shock wave, reflection, bypass and runup during interaction between granular flow and obstacles on steep terrains. A series of numerical simulations concerning granular flows interacting with an array of tetrahedral obstacles of different distributions were conducted. A new dimensionless index called deflection efficiency was proposed, and the effects of tetrahedral obstacle arrays on the flow distance and lateral spreading characteristics of granular flow were quantitatively evaluated. A single tetrahedral obstacle plays a role of dissipation and deflection on granular flow, the latter of which even more obviously changes the granular flow pattern. An array of tetrahedral obstacles shows a comprehensive action of dissipation and deflection on granular flow, where multilevel actions dissipate energy in granular flow through bow shocks, and the splitting and changing actions on the flow path deflect granular flow. The obstacle system could control the final deposit to produce a protection region downstream.