Debris flow is a common geological disaster with spectacularly rapid velocity and agitated state. It may usually cause catastrophic destructions to infrastructures and loss of human lives along the flow path. Thus, slit dams, as a type of open-check dam structures, are widely constructed in mountainous areas to mitigate such destructive flows. In this paper, a numerical model by Discrete Element Method (DEM) was employed to investigate the influence of relative post spacing (b/d max) of slit dams on the debris-dam interactions. The runout distance and the impact forces exerted by debris flows on the slit dams were analyzed. The numerical results show that the impact force decreases with b/d max, whilst the runout distance increases with b/d max. Furthermore, the ratio of peak normalized kinetic energy of all granular materials to that of granular materials passing through the slit dams were analyzed to quantify the regulation function of slit dams on debris flows. The obtained numerical results can provide insights into the optimization of relative post spacing.