Internal charging effect is one of the main factors affecting on-orbit performance of spacecraft. Current measurement can directly obtain the firsthand data of the internal charging effect. It also can obtain electron environment status through current measurement. Based on the existing current data and the possible exposure of energetic electron storm， the current of the internal charging effect is 10 fA—500 pA on the medium and high earth orbits. For the requirement of the current monitoring for the future earth orbit， the Jupiter and Mars exploring mission， a pico current measurement method has been proposed. A pico current monitor unit has been developed and calibrated in the laboratory. The calibration results have shown that the linearity between input current and output voltage is good from 10 fA to 500 pA.