To implement a driving power measurement technique that can be easily integrated in existing high-intensity focused ultrasound (HIFU) systems, to indirectly monitor the acoustic power output by the measurement of the driving power, and then to ensure the efficacy and the safety of the HIFU thermal ablation, an on-line power monitor was designed based on C5948 dual directional coupler and AD8363 root mean square power detector. The equipment was composed of power coupling module, power detecting module and data acquisition module. It was connected in HIFU system to measure the forward and reflected powers as well as the net power using signals of different frequencies with the peak-peak voltages ranging from 20 mV to 200 mV. There are two existing main methods of power monitoring in the HIFU system, i.e. measuring the voltage, current and phase difference of the HIFU transducer as well as using the commercial power meter. The proposed equipment was compared with the above two methods. Results demonstrated that the errors of measuring forward and reflected powers were lower than 10% using the proposed equipment. The measurement error of actual loaded power was lower than 5%, and the errors might come from the directivity of the used coupler.