The novel task-specific room-temperature ionic liquid 3-(N, N-dimethyldodecyl ammonium)propanesulfonic acid hydrogen sulfate ［DMDAPS］·［HSO₄］ was designed and synthesized as a biodegradable recyclable catalyst for one-pot three-component Mannich reaction in water. Twelve β -amino carbonyl compounds were obtained with yield of 80%-91% under the mild conditions. The product can be simply separated from the catalyst/water, and the enviromentally benign catalyst can be reused at least 9 times without noticeably decreasing of the catalytic activity.