Debris flows caused by moraine-dam failure are common on the Qinghai-Tibet Plateau in China. Peak discharges of debris flows should be predicted to evaluate the risk to people and property in valleys below the moraine dams. On the basis of the critical wave method, we have reduced a new series of formulas about the peak discharge of debris flow, including the peak discharge () at the outlet of a failed moraine-dammed lake: or , and the maximal height of the flow below the dam ():. The calculated peak discharge agrees well with the estimate based on the field data at Midui Gully, Tibet. The critical wave method may be applicable to moraine dams in other areas, including the entire Qinghai-Tibet Plateau region and the European Alps.