Imaging　sonar　can　precisely　measure　the　sizes　of　some　large　defects　on　the　surface　of　underwater　concrete　components　(UCCs).　However,　it　is　challenging　for　image　sonar　to　measure　the　widths　of　small　cracks　on　UCCs.　Another　difficulty　is　the　coverage　of　aquatic　organisms,　such　as　moss.　The　moss　coverage　divided　into　four　stages　changes　the　hardness　and　roughness　of　UCCs　to　cause　wrong　signal　intensity　of　echo,　crack　image　generation,　and　crack　width　measurement.　This　article　first　illustrates　the　imaging　principles　of　surficial　cracks　in　UCCs　using　a　two-dimensional　imaging　sonar　and　derives　a　theoretical　equation　of　crack　width　measurement.　Then,　the　appropriate　settings　of　two　main　factors　in　actual　inspection,　scanning　distance　l　and　pitch　angle　alpha,　are　determined　by　experimentally　obtaining　the　measurement　error　distribution　of　concrete　cracks　with　various　stages　of　moss　coverage.　It　also　summarizes　the　correction　equation　of　crack　width　measurement　at　different　stages　of　moss　coverage.　For　the　case　of　unknown　stages　of　moss　coverage,　the　peak　gray　P　and　the　mean　value　M　of　fitting　the　Gaussian　curve　of　the　gray-level　histogram　of　the　sonar　images　are　experimentally　proved　to　be　effective　in　distinguishing　the　different　stages　of　moss　coverage　by　a　K-means　clustering　process.　Experimental　results　indicate　that　0.75　m　＜=　l　＜=　2.0　m　and　15　degrees　＜=　alpha　＜=　30　degrees　is　the　appropriate　ranges　of　l　and　alpha　in　the　practical　measurement.　The　boundary　line　equation　to　separate　Stage　I　and　Stages　II　&　III　of　moss　coverage　is　60　＜=　P　+　M　＜=　80.　The　boundary　line　equation　to　separate　Stages　II　&　III　and　Stage　IV　is　75　＜=　P　+　M　＜=　110.