Based on the experiment, deposit principle of debris flow when it confluence with the main river is studied. Deposit quantity and region is analyzed. According to the experiment, deposit quantity will increase with the increasing of discharge ratio. As deposit speed is considered, it will be first up then down. Deposit quantity will get the maximum value at discharge ratio of 0.5 and get minimum value at 0.65. The curve (relation between deposit quantity and discharge ratio) will be an uphill sinusoid. Deposit region will change little in all experiment conditions. As shown in the experiment, deposit region may expand with the increasing of the confluence angle.