Plasma cleaning has been broadly applied to remove impurities and contaminants from flexible substrates. The equivalent resistance of a plasma load is a very important factor for power measurement. However, the conventional Lissajous figure method requires an external capacitor serially connected to the plasma load, which conducts the DC bias and leads to large measurement deviation. Moreover, conventional plasma cleaning power supplies adopt a high turns ratio but a low-efficiency transformer to drive the plasma load. This study proposes an accurate plasma impedance measurement methodology that does not require an external capacitor and applies it to a parallel resonant full-bridge inverter. A digital signal processor is used to derive the equivalent impedances and estimate the output power and efficiency of the plasma load to achieve constant power control for a plasma power supply system. Based on experimental results, the calculated efficiency is very close to measured efficiency. A prototype inverter under phase-shift control is found presenting efficiency of 88.1% at a full load of 3 kW and 81.5% at a light load of 1 kW respectively.