An ultrasound welding system is basically a piezoelectric structure resonator working at high power. It usually induces heat and results in high temperatures and hence changes the impedance-frequency characteristic. Therefore, a control method to track the resonating frequency becomes very important in stabilizing the ultrasound welding system. In this research, we provide a new control method using a FPGA chip to sweep and catch frequency of the high-power amplifier for especially an ultrasonic apparatus or system. The FPGA basically have two circuits. One circuit is to sweep and catch the working frequency. The other circuit is to adjust the oscillator frequency simultaneously. This control method was implemented on a practical ultrasound welding system currently used in industries. The result proves that the new method can effectively control and automatically rack a proper frequency for the ultrasound welding system.