A　magnetic　quasi-static　(MQS)　surface　integral　equation　(SIE)　formulation　based　on　the　cut　set　analysis　(CSA)　is　proposed　to　extract　parameters　of　interconnects　in　packages.　The　surface　impedance　approximation　is　used　to　describe　the　surface　current　caused　by　skin　effect　at　high　frequencies.　To　accurately　model　arbitrarily　shaped　structures,　triangle　meshes　are　selected　to　discretize　surfaces　of　interconnects　in　the　proposed　formulation.　After　physically　interpreting　the　discretized　matrix　equation　as　a　circuit,　the　CSA　is　tailored　to　carefully　apply　the　charge　conservation　condition.　Triangles　on　each　terminal　are　bounded　together　as　a　supernode　to　enforce　currents　flowing　into/out　interconnects.　In　addition,　an　efficient　preconditioner　and　the　precorrected　Fast　Fourier　Transform　(pFFT)　are　used　to　accelerate　the　convergence　and　matrix-vector　product.　Four　numerical　examples　were　carried　out　to　validate　the　effectiveness　by　comparing　it　with　the　industrial　solver.　Our　results　show　that　the　proposed　formulation　is　accurate,　efficient,　and　flexible　to　model　complex　interconnects　at　high　frequencies.