In　order　to　solve　the　problem　of　poor　real-time　performance　of　traditional　algorithms　and　high　false　alarms　when　there　are　complicated　cloud　disturbances　in　the　scene,　this　paper　proposed　an　infrared　small　target　fast　detection　algorithm　in　the　sky　based　on　the　human　visual　system.　At　first,　a　local　contrast　measurement　method　with　single　scale　was　proposed　to　quickly　calculate　candidate　targets.　Then,　the　Laplace　Gaussian　scale　space　theory　is　introduced　to　calculate　the　multi-scale　filter　response　of　the　candidate　targets.　According　to　the　difference　of　the　statistical　characteristics　of　the　response　value,　the　real　target　is　obtained　through　adaptive　threshold　segmentation.　Experimental　results　show　that　the　proposed　method　has　a　detection　rate　of　up　to　97%,　and　a　false　alarm　rate　of　lower　than　3%,　and　our　method　can　calculate　the　target　position　within　5ms　for　a　256×200　image.　©　2018　IEEE.