Abstract Effective absorption of low-frequency waterborne sound with subwavelength absorbers has always been a challenging work. In this paper. we derive two theoretical requirements for broadband perfect absorption of low-frequency waterborne sound by ultrathin acoustic metasurface under a finite-thickness steel plate followed by semi-infinite air. Based on the theoretical requirements. https://www.lightemupsequences.com/quick-find-Our-Seasons-The-World-in-Winter-Spring-Summer-and-Autumn-best-save/