Aiming at addressing the gap in mechanized transplanting technologies and equipment for the characteristic aquatic vegetableSEleocharis dulcisS(water chestnut) seedlings in field transplanting operations, this study developed a U-shaped hanging cup transplanter forSEleocharis dulcisSseedlings. Structural design and analysis were conducted on key components, including the floating plate frame assembly, U-shaped hanging cup rotary planting mechanism, and furrow-opening and soil-covering device. By establishing a compound kinematic model for the U-shaped hanging-cup rotary planting device, this study analyzed the compound motion trajectory of the hanging cup"s overall carried motion and its opening-closing motion, thereby verifying the feasibility of its planting operation and the reliability of its motion performance. Theoretical analysis of the seedling transplanting process clarified that the main factors affecting transplanting qualification rate were the motion characteristic parameter, opening-closing degree of the hanging cup, and spacing between the furrow opener and soil-covering device. A prototype was fabricated, and single-factor experiments combined with Box-Behnken central composite design experiments were carried out, with transplanting qualification rate (Q) as the response index and the motion characteristic parameter (λ), opening-closing degree of the hanging cup (θ), and spacing between the furrow opener and soil-covering device (S) as experimental factors. The results showed that the theoretical optimal transplanting qualification rate reached 87.5% when λ was 1.425, θ was 45°, and s was 440 mm. Field test results demonstrated that the average transplanting qualification rate of the entire machine was 85.8%, the plant spacing qualification rate was 90.2%, and the depth qualification rate was 75.0%. This research provides a reference for the development of seedling transplanting technologies and equipment.