In　order　to　solve　the　problem　of　underdetermined　blind　source　separation　(BSS)　in　the　diagnosis　of　compound　fault　of　roller　bearings,　an　underdetermined　BSS　algorithm　based　on　null-space　pursuit　(NSP)　was　proposed.　In　this　algorithm,　the　signal　model　of　faulty　roller　bearing　is　firstly　used　to　construct　an　appropriate　differential　operator　in　null　space.　With　the　constructed　differential　operator,　the　mixed　signals　collected　by　the　vibration　sensor　are　decomposed　into　a　series　of　stacks　of　narrowband　signal　containing　the　characteristics　of　faulty　bearing.　Finally,　the　underdetermined　problem　is　transformed　to　an　overdetermined　problem　by　combining　the　narrow　band　signals　and　the　original　mixed　signals　into　a　new　group　of　observed　signals.　In　this　way,　the　separation　of　the　mixed　signals　can　be　realized.　Experiments　and　engineering　data　analyses　show　that　the　problem　of　underdetermined　BSS　can　be　solved　effectively　by　this　approach,　and　then　the　compound　fault　of　the　roller　bearing　can　be　separated.