Abstract: Water inrush is one of the main geological disasters in the tunnel construction in karst areas. A numerical analysis model was established to investigate the flow characteristics of the disaster medium in the filled karst tunnels after water inrush. The migration process of the inrush material in the tunnel was studied, and the effects of water inrush pressure and viscosity of the inrush medium on its flow characteristics were analyzed. The results indicate that the migration process of water inrush in the tunnel can be divided into three stages: jetting into the tunnel, before gushing out of the tunnel entrance, and after gushing out of the tunnel entrance. The viscosity of the medium has a significant impact on the later stage of gushing out of the tunnel entrance. The movement and velocity changes of water inrush objects in the tunnel are mainly driven by pressure, with the velocity being the highest near the water inrush inlet, followed by the entrance, and the middle part of the tunnel being the smallest. Generally, it increases with the increase of water pressure and decreases with the increase of viscosity. The higher the viscosity of the water inrush, the longer the duration from the occurrence of water inrush to the gushing out of the tunnel, but the duration of water inrush decreases with the increase of water inrush pressure. The relationship between water inrush duration and medium viscosity can be fitted by Exponential function. The greater the water inrush pressure is,the faster the water inrush duration increases with the increase of viscosity. The research results could enrich the researches of flow characteristics of water inrush in the tunnels and provide certain reference and theoretical basis for disaster prevention and rescue.