The three-phase three-dimensional star-shaped iron core transformer has the advantages of light weight, compact structure, balanced three-phase magnetic circuit and balanced output load. Its three-phase iron cores are 120° apart and have no central column, showing great potential for lightweight design. Based on the electromagnetic theory and design method of transformers, the three-dimensional star-shaped iron core structure of the three-phase rectifier transformer is optimized for ultra-light and compactness. Based on the core structure composition and lamination method, a three-dimensional star-shaped iron core magnetic circuit model and magnetic equivalent circuit are established, and the electromagnetic field control equations and boundary conditions of the three-phase three-dimensional star-shaped iron core transformer are derived. A three-dimensional electromagnetic field-circuit coupling finite element model of the three-phase three-dimensional star-shaped iron core transformer is constructed. Based on the logic analysis of the Maxwell finite element program, a nonlinear model of magnetic anisotropy of silicon steel sheets is constructed. The influence of the balance of the three-dimensional star-shaped structure on the no-load performance and leakage magnetic distribution of the transformer is discussed. The research results show that the three-dimensional star-shaped iron core has significant advantages in weight, magnetic circuit balance, magnetic saturation and iron loss.