The invention discloses a method for hardening enamelled flat wire on line, which comprises the following steps of: (1) determining elongation of a copper wire according to required hardness; (2) calculating size of a die according to the elongation; (3) calculating feeding size of the copper wire according to the elongation; (4) putting the die the size of which is calculated on a drawbench and adding wire drawing liquid for drawing wire; (5) annealing the cooper wire obtained by the wire drawing of the die in an annealing furnace at high temperature of 200-400 DEG C for eliminating the wire drawing liquid on the surface of the copper wire and the inner stress of copper; and (6) painting and baking the copper wire obtained by annealing to obtain a enamelled flat wire with required hardness. By corresponding elongation, a copper conductor become hard through increasing density by wire drawing and meets the requirement on hardness, and the maximum hardness can reach 240N/mm; and since the die is used, the size and the hardness of the enamelled flat wire are quite uniform, and the elongation and the electrical conductivity of conductors are improved.