A semiconductor device and a forming method thereof are provided; the method comprises the following steps: providing a substrate; forming a source drain material layer on the substrate and a mask layer on the source drain material layer, wherein the mask layer has a first groove that exposes partial the source drain material layer; forming a protection layer on the side wall of the first groove;using the musk layer and the protection layer as a mask to etch the source drain material layer on the bottom of the first groove, thus forming a second groove penetrating the source drain material layer in the source drain material layer; removing the protection layer after the second groove is formed; removing the protection layer so as to form a channel material layer and a grid structure on the channel material layer, wherein the channel material layer is located on the side walls and bottoms of the first and second grooves; removing the mask layer after the channel material layer and thegrid structure are formed. The method can improve the semiconductor device electricity performance stability.