The invention provides a method for calculating length of wire requiring tensioning for overlarge sagging arc distribution network wires. The method comprises following steps:(1) measuring the length s of a distribution network wire between two poles, the horizontal distance l between two poles, and height difference h between two hanging points; (2) deducing an arc equation for overlarge sagging arc wires; (3) obtaining stress for the overlarge sagging arc wires, and determining whether the wire can be tensioned according to a standard; (4) calculating corresponding target arc equation according to input target arc sagging value, and performing tension calculation on the equation after calculating, and judging according to the standard to determine whether wire tensioning is effective; (5) calculating the length of wire to be tensioned, and obtaining wire length of target arc wire according to the target arc equation. The method of the invention can realize automatic calculation by computer software. The invention can be applied to the wire tensioning construction of distribution network, and has the advantages of less measuring data, simple operation and convenient operation, and can provide great convenience for the distribution network maintenance personnel.