The invention relates to a coarse fiber for concrete, which is in a shape formed by spirally twisting a polygonal prism along the axial direction, wherein the length of the coarse fiber is 20-60 mm and the diameter or the equivalent diameter of the section is 0.10-1.20 mm; and the surface between adjacent edges is concave. The preparation method comprises the following steps of: drying preparation materials of the fiber to constant weight and adding the materials into a double-screw extruder; after extruding through a spinneret plate hole, cooling filaments by utilizing cooling liquid; then connecting the filaments into a guide wire hole plate which is the same as the spinneret plate hole in shape and is rotated for an angle; carrying out grading multi-time stretching on the filaments of the guide wire hole plate; after the stretching is finished, carrying out heat setting; and then, cutting according to the length and packaging to obtain the product. According to the invention, through changing the appearance and the shape of the fiber, a good interface acting force exists between the coarse fiber and a substrate and the rigidity of the fiber is increased, so as to enhance or improve the performance of the coarse fiber in enhancing the anti-cracking, anti-breaking, tough, anti-impact properties and the like of the concrete.