Original　fracture　structure　in　brittle　rock　mass　usually　has　a　bad　influence　on　the　safety　and　stability　of　artificial　underground　structures.　The　former　laboratory　test　results　proved　that　the　secondary　failures　on　the　surface　or　inside　the　samples　showed　different　development　procedures　and　distributions　when　a　3D　internal　open-type　preset　flaw　was　inside　the　sample.　However,　researchers　cannot　obtain　the　secondary　cracks＇　accurate　failure　mode　and　initiation　mechanism　in　experimental　tests.　Accordingly,　the　discrete　element　numerical　simulation　(PFC3D　software)　is　applied　in　this　study　to　reproduce　the　laboratory　tests.　The　main　results　are　as　follows:　1)　The　2D　through　preset　flaw　strongly　influences　the　state　of　the　surface　displacement　field,　the　crack　distribution　near　the　surface,　and　the　initiation　strengths　of　secondary　tensile　and　shear　cracks.　2)　When　a　3D　internal　preset　flaw　in　the　sample,　the　local　failures　initiated　from　the　internal　3D　flaw　would　keep　hiding　during　the　majority　period　of　the　test.　3)　By　applying　the　particle　displacement　trend　method,　it　is　evident　that　rock　adjacent　to　the　inclination　boundary　of　the　internal　flaw　suffers　＂Mode　II＂　shear　failure;　rock　near　the　strike　boundary　of　the　3D　flaw　presents　＂Mode　II　&　III＂　shear　failure.