The invention discloses a DC isolation transformer with a same-iron-core expandable structure. The DC isolation transformer is composed of a number of sub transformers connected in a cascade form. Each sub transformer includes an iron core assembly, an input side winding and an output side winding. Each iron core assembly consists of a first iron core column and a second iron core column. Each input side winding includes an input side inner winding, an input side insulating support and an input side outer winding casing which sleeve the corresponding first iron core column in sequence, and theinput side outer winding casing contains an input side outer winding. Each output side winding includes an output side inner winding, an output side insulating support and an output side outer winding casing which sleeve the corresponding second iron core column in sequence, and the output side outer winding casing contains an output side outer winding. The two ends of the input side inner windings and the two ends of the output side inner windings are connected respectively to form an energy transmission loop. By adopting the sub transformers with the same iron core, double insulation and isolation capability can be realized under the same core. Moreover, the sub transformers can be expanded by cascading to achieve insulation and isolation at a higher voltage level.