A sheet processing device includes a supporting plate, a stapler, a casing, a protrusion structure, and a swinging arm. The casing is disposed over the supporting plate. A sheet conveying channel is formed between the supporting plate and the casing. The protrusion structure is disposed on a lower surface of the casing, located beside the stapler and accommodated within the sheet conveying channel. During the process of introducing a sheet into the supporting plate, the stapled side of the sheet is flattened by the protrusion structure, and thus the possibility of upturning the sheet is minimized. During the process of introducing the plural sheets to the stapler, the stapled sides of the plural sheets are flattened by the protrusion structure, and thus the formation of the folded corners of the sheets is avoided.