When target water level falls sharply in river model experiment, tail gate may open excessively and flow velocity increases in a short time, destroying the riverbed terrain. In view of this, a steady control method for tail gate is put forward: stopping the tail gate PID (proportion-integration-differentiation) control mode in the early stage of experiment until the modeling water level approximates the target water level. In the scenario, if the target water level drops more than forecast, the target would decline in a regular interval until the actual water level is close to the final target .The magnitude and duration of the target decline are different according to different external experiment. This scheme is applied to the water supply system of Yangtze River Flood Control Model, and is embedded into the PID control system of tail gate. Experiments verified that the excessive adjustment of tail gate when target water level drops sharply is effectively controlled, hence ensuring the steady water level and improving the reliability of experiment system. As the duration of water level control is increased, we should select proper control parameters according to the model features.