This study investigates and analyzes an ultra-wideband water-based metamaterial absorber. The unit cell consists of a shell, a water layer, and a metal backplane, with the shell fabricated from photosensitive resin and the water layer comprising an ‘X′ shaped body and four spheres. Designed for the millimeter wave band, simulation results indicate that the absorber′s performance exceeds 90% absorption within the frequency range of 22.7 GHz to 63.9 GHz when illuminated by vertically incident electromagnetic waves. Perfect absorption is achieved within the frequency range of 39.5 GHz to 44.4 GHz. The absorber exhibits polarization insensitivity and wide-angle incidence characteristics. Additionally, equivalent dielectric theory is applied to analyze the equivalent impedance of the absorbing material. Given the unique absorption frequency band, the proposed ultra-wideband water-based metamaterial absorber holds significant potential for applications in millimeter wave communication systems.