In　czochralski　silicon　with　large　diameter　production,　in　order　to　improve　the　quality　of　crystal,　a　cusp　magnetic　field　is　often　used　to　reduce　melt　convection.　In　this　paper,　Numerical　method　is　used　to　analyze　how　coil　radius,　the　distance　between　coils　and　the　relative　position　between　equal　gauss　surface　and　freedom　surface　effect　melt　convection,　temperature　distribution　and　the　oxygen　concentration　distribution　at　growth　interface.　Results　indicate　that　keeping　the　same　magnetic　field　strength　at　the　center　point　of　the　melt　bottom,　the　radius　of　the　loops　and　the　distance　between　the　loops　have　little　effects　on　the　flow,　the　temperature　and　oxygen　concentration.　When　the　equal　gauss　surface　is　under　the　freedom　surface,　the　oxygen　concentration　is　lower　than　that　equal　gauss　surface　is　equal　to　the　freedom　surface.　This　is　agreed　with　the　experiment.