The invention discloses a root-cutting land leveller for improving deteriorated grassland and an improvement method thereof. The root-cutting land leveller comprises a frame, a transmission system, a hydraulic system, a root-cutting device, a land levelling device, a pressing device and a suspension device, wherein the root-cutting device is arranged on the front part of the frame; the suspension device is arranged above the frame; the land levelling device and the hydraulic system are arranged on the back part of the frame; the pressing device is connected with the back part of the frame. After the end of alfalfa harvesting on perennial alfalfa grassland and before the autumn of a current year or the spring green returning in the next year, root-cutting land levelling operation, including a root-cutting operation procedure and a land levelling operation procedure is performed. Root-cutting, land levelling and pressing operation procedures are combined, so that underground aged root systems of perennial alfalfa grassland can be cut off, and the forage yield is increased, deteriorated grassland hardened soil is damaged, grassland levelling, pressing and stumping after cultivation are realized, the soil aeration and soil water storage and preservation capabilities are improved, the operation cost is reduced, and the working efficiency is increased. The root-cutting land leveller belongs to the field of stockbreeding machines.