An　ultrafast　and　convenient　method　for　visually　detecting　CaMV35S　promoter　amplicon　(amplified　products)　was　established　by　using　CRISPR/Cas12a　system　coupled　with　a　designed　reaction　vessel.　Genetically　modified　(GM)　soybean　(Roundup　Ready®)　powders　containing　CaMV35S　promoter　were　employed　as　detection　targets,　which　were　amplified　by　loop-mediated　isothermal　amplification　(LAMP).　The　CRISPR/Cas12a　system　directly　mixed　with　amplified　products　at　37　°C　for　5　min　and　detection　results　could　be　clearly　identified　by　the　naked　eye　under　UV　light　(254　nm).　A　designed　reaction　vessel　was　employed　to　make　operation　easier　and　could　effectively　prevent　contamination　at　the　source.　The　CRISPR/Cas12a　detection　system　was　optimized　in　our　study　and　the　concentration　of　magnesium　ions　was　proved　to　be　important　for　the　work　of　CRISPR/Cas12a　system.　The　optimized　concentration　range　of　magnesium　ions　was　between　10　mM　and　12　mM.　Besides,　the　activated　Bst　DNA　polymerase　also　had　little　effects　on　CRISPR/Cas12a　system.　The　developed　method　could　significantly　distinguish　the　specific　and　non-specific　amplification.　And　as　low　as　0.05%　transgenic　contents　in　soybean　powders　could　be　detected.　It　would　have　the　potential　to　be　complementary　to　instrument-based　ultrahigh　sensitive　method　and　provide　a　new　solution　for　on-site　rapid　detection.　©　2019　Elsevier　B.V.