The present invention relates to a parameter relation-based alternating current line length identification method. The method is characterized by comprising the following steps that: 1) the electrical parameters of each alternating current line in a power grid plan are obtained and subjected to data preprocessing, wherein the electrical parameters include reactance, electrical susceptance, the number of split wires, geometric mean distance and conductor radius; 2) the per-unit-length reactance and electrical susceptance of a certain line in the alternating current lines are calculated according to the electrical parameters which have been subjected to the data preprocessing; 3) reactance and electrical susceptance corresponding to the line are searched from the step 1); 4) the length of the line is calculated according to the calculated per-unit-length reactance and electrical susceptance of the line as well as the reactance and electrical susceptance of the line; 5) rationality judgment is performed on the obtained length of the line; 6) operation from the step 2) to the step 5) is performed on all lines in the alternating current lines; and 7) the length identification results of all the alternating current lines are outputted. The method of the present invention can be widely applied to the power system planning field.