Abstract: In atomic oxygen ground simulation equipment, a current source is used to constrain the emittance of atomic oxygen by utilizing magnetic mirror effect. The current source is also used to calibrate the atomic oxygen parameter measuring instruments. A precision voltage-controlled current source based on the op amp is developed. In order to mitigate the influence of the power supply ripple, a switching power supply filter is developed. The testing results show that the switching power supply filter can reduce the switching power supply noise effectively. The ripple coefficient of +15 V power supply drops to millionth. The constant current source can output a current from 10 pA to 1 A, whose accuracy is 500 fA. Its load stability is excellent.