Abstract: Recently，the drag-free satellite mission needs more precise performance for the cusped field thruster （HEMPT），such as precise thrust resolution and accurate measurement of thrust noise. However，due to the technical limitations，the current three-wire torsion pendulum measurement system cannot quickly and accurately reflect these indicators. In this paper，an equivalent method for calculating the thrust from the data measured by the probe is deduced，and is verified by experimental measurements. The results show that the proposed method can calculate the thrust precisely in a certain range. So the method provides a reference for the measurement of the mic-Newton thrust and the thrust resolution.