The　construction　industry　is　an　important　industry　supporting　social-economic　development.　Detecting　cracks　in　walls　could　help　to　maintain　the　conditions　of　buildings.　A　wall　crack　detection　algorithm　based　on　adaptive　double　threshold　greyscale　transform　is　proposed.　The　MATLAB　built-in　‘graythresh’　function　is　modified　in　order　to　have　a　flag　bit　to　preliminarily　selection　of　the　greyscale　transformation　threshold　where　the　grey　transform　threshold　and　the　binarization　threshold　can　be　automatically　adjusted　based　on　the　image　processing　effect　using　a　preliminaryselected　threshold.　The　algorithm　enhances　the　crack　information　and　weakens　the　background　information　by　multiple　morphological　operations　of　removing　isolated　small　area　pixels.　The　MATLAB　simulation　results　show　that　the　proposed　algorithm　has　an　accuracy　of　96.16%,　time　from　inputting　the　image　to　the　completion　of　the　labelling　is　3-15　seconds,　which　depends　on　the　complexity　of　the　crack.　©　The　2023　International　Conference　on　Artificial　Life　and　Robotics　(ICAROB2023).