The invention discloses a forming method of a semiconductor device. The forming method comprises the steps of providing a to-be-etched material layer; forming sacrificial layers on the to-be-etched material layer; forming side walls on the lateral walls of the sacrificial layers; after forming the sacrificial layers, forming compensation layers on the exposed to-be-etched material layer; after forming the compensation layers and the side walls, removing the sacrificial layers; and after removing the sacrificial layers, etching the compensation layers and the to-be-etched material layer by adopting the side walls as masks. The compensation layers are formed on the exposed to-be-etched material layer after the sacrificial layers are formed and before the sacrificial layers are removed, so that the height difference between corresponding material surfaces at two sides of each side wall is relatively small after the sacrificial layers are removed, and the height differences between recesses in a target layer are relatively small, thereby improving the performance of patterns in the semiconductor device.