In order to expand the solid angle for imaging of electrons in ion-atom collisions,　we designed a complex Helmholtz coils composed of four single coils. Theoretical simulations were carried out to optimize the arrangement of the coils. The complex is constructed according to the theoretical analysis, and the magnetic fields were measured for interested regions. The measured results show that the relative uniformity of the magnetic fields is ±0.6%, which satisfies the requirements of collision experiments.