Abstract: Based on a 1:30 large geometric scale for the hydraulic model， the hydraulic tests are made according to the gravity similarity criterion. The test results show that under the overall design flood， the original design and modification scheme will not cause the erosion of downstream river channel. Flow in downstream channel is flowing smoothly， and the velocity distribution is uniform. During the period of the check flood， the unit width discharge increases with the increase of flood discharge along the spillway. Air entrained energy dissipation effect reduces along the spillway of flow surface in the original design， which causes serious scour along the lower channel by the nappe and loads to the tunnel outlet channel blocked seriously. It will affect the safety of the tunnel during flood discharging. Through the scheme optimization modification， the ladder step size of 0.90 m×0.72 m with continuous flip bucket scheme is put forward. The energy dissipation is higher under the check flood condition， the nappe distance and the scour degree of the lower channel are reduced greatly， the downstream flow is flowing smoothly and the velocity distribution is uniform along the channel. The flow velocity in the lower channel is less than 2.80 m/s and meets the requirements of the safety flood discharge from the stepped overflow dam.