Abstract: As an efficient, energy-saving and environment-friendly welding technology, ultrasonic welding has been widely used in metal, plastic, non-woven fabric welding and other fields. In metal welding, it needs to accurately control the welding time, power, energy and other parameters, therefore the design requirement of power supply is very strict. A digital ultrasonic metal welding power supply was designed. The system adopts direct digital frequency synthesis technology, automatically tracks the resonant frequency of the transducer according to the phase difference of voltage and current at the acquisition output terminal. The power supply can automatically adjust its waveform frequency, which maximizes the output power and keeps the welding power as the same each time. At the same time, the system adopts the method of additional amplitude compensation at the first welding to solve the problem of energy reduction caused by frequency change. The system can adopt three welding modes of external control mode, time mode and energy mode to meet the requirements of different welding processes.