An oil-immersed transformer is a new, high-performance transformer with a more rational structure and superior performance. Its three-dimensional rolled iron core, with three core columns arranged in an equilateral triangular pattern, has no air gap in its magnetic circuit. The winding is tighter, with all three magnetic paths having the same length. The cross-sectional area of the iron core columns is closer to circular, thereby further enhancing performance, reducing losses and noise, achieving three-phase balance, and minimizing the third harmonic components. This product is particularly suitable for rural and urban, industrial and mining enterprise grid renovations, as well as for transformers used in modular transformers and pre-assembled substation transformers.