The　design　theory　and　method　of　formed　milling　cutter　for　double-helix　screw　of　progressing　cavity　pump　are　presented.　Through　analyzing　the　shape　and　characteristic　parameters　of　double-helix　screw,　the　helicoids　equation　and　axial　curve　equation　of　double-helix　screw　were　established.　According　to　the　relative　position　relations　between　formed　milling　cutter　and　double-helix　screw　in　the　machining　process,　the　geometric　mapping　relationship　of　screw　coordinate　system　and　formed　milling　cutter　coordinate　system　was　established　by　using　the　coordinate　transformation　theory.　Based　on　noninstantaneous　envelope　method　and　the　meshing　conditions　between　formed　milling　cutter　and　double-helix　screw,　the　contact　line　equations　were　established　by　minimum　value　method.　By　analyzing　the　machining　errors　caused　by　resharpening　the　formed　milling　cutter,　the　tooth　back　curve　equation　was　established　based　on　spiral　of　Archimedes,　and　the　profile　equation　of　formed　milling　cutter　with　constant　back　angle　was　got.　On　this　basis,　the　formed　milling　cutter　of　processing　double-helix　screw　was　designed,　and　the　cutter　head　and　tool　post　were　manufactured,　respectively.　The　measuring　results　have　shown　that　this　method　can　satisfy　the　requirements　of　machining　accuracy　for　double-helix　screw.　So　this　is　an　effective　method　to　get　formed　milling　cutter　profile　for　double-helix　screw.