The invention relates to a foil coil of a continuous casting square billet online induction heater. The existing coil of the continuous casting square billet online induction heater is formed by winding a copper tube which has large power consumption and low electrical efficiency. The foil coil comprises copper foils, insulation cushion blocks and cable connection copper bars. The insulation cushion blocks are uniformly put between the copper foils which are wound in a layering manner, an air passage is formed between the insulation cushion blocks, one cable connection copper bar is respectively led out at each of the two ends of the copper foil of an inner layer, another cable connection copper bar is led out at one side of the copper foil of an outer layer, and the copper foils are wound into a hollow square billet to form the coil; and the external periphery of the coil is fixed through adopting an insulation clamping board, and the inside of the coil is coated through adopting a refractory material. The foil coil is adopted, so that the electrical efficiency is high; and air cooling is adopted, and the foil coil is simple in structure.