The invention provides a positioning tool for trimming of an external corner of stone. The positioning tool comprises a handle, a connecting piece, a first trimming plate and a second trimming plate.The connecting piece is arranged in the mode that a first wing plate and a second wing plate are integrally formed with an L-shaped right angle, a first strip-shaped hole and a second strip-shaped hole are further formed in the first wing plate and the second wing plate correspondingly, and a strip-shaped groove is further formed in the corner of the first wing plate and the second wing plate; thehandle is fixedly arranged on any one of the outer side faces of the connecting piece; the first trimming plate and the second trimming plate each comprise a first inclined plane and a second inclined plane, wherein the second inclined plane and the first inclined plane are arranged oppositely and located on the same plane; a first fastener and a second fastener are further welded to the first trimming plate and the second trimming plate correspondingly; and the first fastener penetrates through the first strip-shaped hole and can be used for adjusting and locking the position of the first trimming plate, and the second fastener penetrates through the second strip-shaped hole and can be used for adjusting and locking the position of the second trimming plate. The quality defects that tooth breakage and damage of the part, at the corner body of the external corner, of the stone occur, and an abutted seam at the corner body of the external corner of the stone is nonuniform and irregularare overcome.