A　modulation　system　of　Stokes　parameters　was　developed　based　on　the　magneto-optical　effect.　The　magnetic　curves　and　driving　characteristics　of　the　magneto-optical　modulator　are　analyzed.　The　magneto-optical　modulator　was　digitally　controlled　by　constant　current　source.　This　system　has　been　successfully　applied　to　the　continuous　variable　coherent　optical　communication　in　the　free　space,　which　eliminates　the　effects　of　magnetic　stranded　and　saturation,　and　its　modulation　current　is　linear　proportion　to　the　magnetic　induction.　Compared　with　the　conventional　driving,　the　use　of　the　constant　current　source　driving　is　faster　about　5　times.