In power ultrasonic applications, precise amplitude control is an important factor for the processing quality. Traditional detection control schemes require complex external instruments or detection circuits, which may have problems such as poor flexibility and nonlinearity. To solve this problem, this paper implements a lightweight amplitude detection and control scheme. A high-speed analog-to-digital (AD) is used to sample the current and voltage of the transducer. The dynamic branch current is calculated in real time which can reflect the amplitude. The field-programmable gate array (FPGA) is used as the ultrasonic power controller to control the transducer operating frequency and the dynamic branch current in the closed-loop. In this design, the ultrasonic power supply operates at the anti-resonant frequency to achieve higher energy transfer efficiency. The experimental results show that the scheme can accurately control and stabilize the amplitude when the load heavily changes. The results of this study provide a lightweight solution for the constant amplitude design of power ultrasonic power supplies.