For large-capacity transformers, the low-voltage winding often adopts single helical windings design. In order to reduce additional losses, transposed wires are widely used. However, further research is needed to investigate the impact of transposition structure of single helical windings on electromagnetic force characteristics. Therefore, this study constructs a finite element model of a transformer with single helical windings based on actual prototype parameters. The field-circuit coupling model is employed to simulate the magnetic field distribution under short-circuit conditions. The results show that the transposition structure of single helical windings distort the magnetic flux density, primarily affecting the transposition area and the adjacent wire cakes. In terms of short-circuit electromagnetic force, there is a significant difference in the radial direction between the two columns of wires in single helical windings, with the outer side of the wire on the right experiencing the greatest radial electromagnetic force. As the distance from the iron core increases, the radial short-circuit electromagnetic force of the wire cake at the transposition structure gradually increases, further exacerbating the non-uniformity of the forces acting on the winding. However, the axial electromagnetic forces are relatively similar. This study is helpful in predicting the mechanical behavior of single helical windings in transformers under multiple short-circuit impacts, and provide guidance for the short-circuit electromagnetic stability verification and operation evaluation of large capacity transformers.