Three-dimensional　(3-D)　reconstruction　based　on　sonar　imaging　is　an　intuitive　and　comprehensible　form　for　describing　an　underwater　defective　pile.　Most　existing　3-D　reconstruction　methods　are　inefficient　as　they　require　the　extraction　of　numerous　feature　points　from　many　sonar　images.　A　rapid　3-D　reconstruction　method　for　underwater　defective　piles　based　on　two-dimensional　(2-D)　images　obtained　using　mechanically　scanned　imaging　sonar　(MSIS)　is　proposed　herein.　First,　the　correspondences　between　the　geometric　features　of　MSIS　images　and　actual　defects　are　established　based　on　the　mapping　relationship　between　actual　object　and　MSIS　image.　Subsequently,　no　more　than　three　feature　points　are　extracted　from　2-D　MSIS　images　to　recognize　the　contour　segments.　The　contour　segments　are　assembled　into　the　entire　cross　section　and　then　into　the　entire　defective　pile.　The　applicability　of　the　proposed　method　is　verified　via　a　3-D　reconstruction　for　a　scaled-down　pile　with　multiple　defect　distributions.　©　2023　Sheng　Shen　et　al.