The invention provides a wide-range current transformer and a gas gap length calculation method. The transformer structurally comprises a primary winding, a secondary winding and iron cores. The iron cores are composed of the fixed iron core and the movable iron core. A gas gap is formed by disassociating the movable iron core with the transformer. The structure is simple, and implementation is convenient. When primary current of the current transformer is large, the magnetizing curves of the iron cores are located in a linear region by adjusting the gas gap length, the problem that because of saturation of the iron cores, errors are large is effectively solved, and iron core loss is reduced. The gas gap length calculation method and a current inductor error calculation formula are clear, and calculation and analysis are convenient.