The invention relates to an oil-immersed iron-core reactor structure. The oil-immersed iron-core reactor structure comprises a box body and an iron core in the box body, insulating oil is arranged inthe box body, and the iron core is completely immersed into the insulating oil in the box body. The oil-immersed iron-core reactor structure is characterized in that a part of air cavity is reserved between the liquid level of the insulating oil and the top of the box body, air is sealed in the air cavity, and the air cavity serves as a space for cold shrinkage and thermal expansion of the insulating oil on the lower portion. According to the oil-immersed iron-core reactor structure, for an oil-immersed iron-core reactor which is low in voltage and small in volume, oil conservator and expanderstructures cannot be adopted any more, but the mode that the box body is properly enlarged and the air cavity is reserved is adopted, so that the functions of an oil conservator and an expander are achieved, and the overall performance of the reactor is not affected. By means of the oil-immersed iron-core reactor structure, the structure of the reactor is simplified, and the cost of the oil-immersed iron-core reactor is reduced.