The invention provides a new method for controlling flying shears for bars and wires based on T400, which is an advanced electrical control method for start-stop type flying shears for bars and wires. The method comprises the steps of accurate shearing-blade position control, shearing length control and automatic shearing optimization, and the like; in the process of production, when an upstream rolling mill has an accident, or the quality of a section of a rolled piece is poor, a fragmentation function can be started on line so as to carry out partial fragmentation or complete fragmentation on the rolled piece, and the fragmentation and shearing speed is in step with the running speed of the rolled piece automatically; and when the fragmentation operation is stopped, a flying shear is automatically positioned to a start position and enters into a standby state of the normal shear mode. In the invention, the powerful computation function of the T400 is fully used, core control parts of the flying shear are completely integrated into the T400, and high-speed counters, shaft positioning modules and touch screens are not required, therefore, the cost is reduced, and high accuracy is obtained; and when the flying shear carries out double-length shearing at a speed of 21 m / s, the double-length error is basically within 75 mm, and partial double-length error can be within 30 mm.