Abstract: An algorithm of left-middle-right difference based on a small area tracking arithmetic is proposed for detecting feeble edge signal in high temperature condition. In a appointed small area, CCD pixel's gray value in vertical orientation is accumulated. The difference between any two of three sequential pixel accumulative total is obtained and the most difference absolute value is regarded as the method's result. The experimental result shows that the mothed is feasible and can detect feeble edge which classical edge mothed can not recognise, and its precision can reach pels level.