The　design　theory　and　method　of　formed　milling　cutter　for　eccentric　screw　is　presented.　Through　analyzing　the　shape　and　characteristic　parameters　of　eccentric　screw,　the　spiral　surface　equation　of　eccentric　screw　was　established.　Considering　the　relative　motion　and　the　meshing　conditions　between　formed　milling　cutter　and　eccentric　screw,　the　contact　line　equation　was　established　based　on　non-instantaneous　envelope　method.　Further,　from　the　cross-section　shape　of　the　rotary　surface　formed　by　the　contact　line　around　the　axis　of　cutter,　the　profile　equation　of　formed　milling　cutter　was　deduced.　By　analyzing　the　causes　of　processing　errors　after　wear　of　formed　milling　cutter,　the　tooth　back　curve　equation　was　achieved　in　accordance　with　spiral　of　Archimedes　and　got　the　profile　equation　of　formed　milling　cutter　with　constant　back　edge　was　gotten.　Finally,　the　interference　for　profile　equation　of　formed　milling　cutter　was　checked.　Simulation　shows　that　the　profile　of　formed　milling　cutter　coincides　with　the　spiral　surface　of　eccentric　screw　and　the　designed　profile　equation　of　formed　milling　cutter　conforms　to　the　requirements　of　the　processing.　This　work　provides　a　theoretical　foundation　for　the　design　and　manufacture　of　formed　milling　cutter.