The utility model discloses a cutting knife for a horizontal shearing machine, comprising a bar-type fixed blade and a bar-type moveable blade which are matched with each other, wherein the cutting edge of the fixed blade comprises a first straight-line segment and a first inclined segment, and the moveable blade comprises a second straight-line segment and a second inclined segment matched with the shape of the cutting edge of the fixed blade. The cutting knife for the horizontal shearing machine has simple structure and skillful design, increases the efficiency of processing U-shaped channel steel and meets the requirements of enterprises on usage.