The invention discloses a system and method for constructing an undersea forest. The system comprises a plurality of seedling clamping ropes and large kelp holdfasts arranged on the seedling clamping ropes at intervals. The upper end of each seedling clamping rope is connected with a floater and/or the lower end of each seedling clamping rope is connected with a sinker. The floater is an aluminum hollow sphere, ellipsoid or cylinder which has buoyancy. The sinker is stone or a metal block which does not have buoyancy. The seedling clamping ropes are corrosion-resistant nylon ropes which are different in length. Through the system, large kelp can be attached to the seedling clamping ropes and the large kelp holdfasts to grow, the seedling clamping ropes have mobility and can be moved as required to the place where the large kelp needs to be cultivated, a large kelp community is recovered, and the economical restoration is achieved.