A　novel　base　force　element　method　(BFEM)　with　separable　interfaces　based　on　the　complementary　energy　principle　has　been　proposed.　The　linear　elastic　fracture　problem　of　mode　I　cracks　is　solved　by　this　complementary　finite　element　method.　The　state　of　the　internal　interface　can　be　divided　into　three　states:　bonded,　damaged,　and　debonding.　The　force　on　the　surface　of　an　element　is　considered　the　basic　unknown　variable　of　BFEM　and　can　be　directly　used　to　determine　the　internal　interface　state.　Interpolation　functions　are　not　required　when　approximating　assumptions　due　to　the　relationship　between　base　force　and　traditional　stress　tensors.　The　complementary　energy　functionals　(CEFs)　of　the　elements　corresponding　to　the　above　three　states　are　established.　The　integral　symbols　of　these　CEFs　are　eliminated　according　to　the　approximation　assumption.　The　control　equations　of　small　deformation　and　large　elastic　deformation　are　obtained　by　taking　stationary　values　for　the　basic　unknown　variables.　The　trust　region　method　of　incremental　format　is　used　to　solve　these　nonlinear　equations　in　the　numerical　calculation　process.　Calculations　show　that　BFEM　is　accurate　and　easily　converged.