The trash rack at the inlet of generating unit of a powerhouse with roof overflow is destroyed off the gate slot suffering from a 20-year event flood. To find out the causes of damage, we simulated the longitudinal flow pattern in the water diversion canal, and analyzed the forces on trash rack according to simulation result. Results suggest that strong transverse vortex and negative pressure in the fore-bay were induced by roof overflow. Affected by long-time longitudinal flow impact and flow pulsation, the trash rack gradually lost strength and experienced deformation, and was ultimately destroyed off the gate slot. We suggest that the impact of longitudinal flow should be considered in the design of trash rack of the same type power station to avoid similar accidents.