The invention relates to a cutting method and a cutting device for crystal bars, aiming at improving the nanometer shape of the surface of cut wafers. The metal wires are simultaneously moved when being provided with cutting fluid, and the feed-in crystal bars are cut into a plurality of wafers by using the cutting fluid, and then working processes such as grinding, etching, polishing are performed. The crystal bar cutting method of the invention comprises a first cutting stage, wherein one side of the crystal bars is cut by using a first metal wire with small diameter so as to form a first cutting part on the wafer; a second cutting stage, wherein the rest part of the crystal bars is cut by using a second metal wire with the diameter larger than the diameter of the first metal wire so asto form a second cutting part on the wafer in succession with the first cutting part.