In　this　paper,　the　research　purpose　is　how　to　measure　the　errors　of　the　gun　barrel　pointing　to　the　target　for　the　towed　anti-aircraft　guns.　The　paper　proposes　an　image　processing　algorithm　based　on　the　local　entropy　to　solve　the　problem　of　the　pointing　error.　First,fix　the　CCD　image　acquisition　device　on　the　gun　barrel　and　ensure　that　its　optical　axis　be　parallel　to　the　barrel　axis.　Second,　the　target　video　images　are　captured　during　the　target　tracking　process　and　are　processed　with　the　algorithm　based　on　the　local　entropy,　and　so　the　errors　of　the　CCD　optical　axis　pointing　to　the　target　can　be　obtained.　At　last,　by　the　parallax　correction,　the　errors　of　the　gun　barrel　pointing　to　the　target　can　be　gotten.　After　the　field　test,　the　algorithm＇s　effectiveness　and　practicality　for　this　device　have　been　validated.