The invention discloses a cultivation method of a multi-root-exposed ornamental tree and a cultivation device thereof. The method comprises the following steps of: setting up the cultivation device of the multi-root-exposed ornamental tree, wherein the device comprises a cylinder internally provided with a partition screen used for supporting soil, and sequentially laying a water storage cotton layer and a cultivation soil layer on the partition screen from the bottom up; arranging an air layer below the partition screen, and arranging a manual regulation opening on the cylinder wall of the air layer of the cylinder; planting a plant sapling in cultivation soil on the top layer of the device; manually trimming, winding and arranging the root of the plant through the manual regulating opening; leading the root of the plant to grow to underground from the bottom of the cultivation device of the multi-root-exposed ornamental tree; and dismounting the cultivation device of the multi-root-exposed ornamental tree after the root of the plant firmly grow underground, so as to enable the upper part of the root of the plant to be exposed above the ground. Compared with the prior art, the invention has the advantages of short cultivation period, high exposed root, attractiveness and practicability.