In the large-scale pulsed power device, the transient strong magnetic field, which is caused by transient high current, may have effects on the insulation property of the whole system. Therefore, in order to investigate the magnetic field space-time distribution and the influence generated by the axial magnetic field on the insulation performance in the pulsed power device, circuit simulation and transient magnetic field simulation are utilized. It can be found that the coupling coefficient makes much influence on the phase difference between the magnetic field and the load voltage. As the coupling coefficient reduces appropriately, the magnetic field in the vicinity of the peak load voltage increases greatly. The magnetic field distribution in the space is much related to the structure of transformers. In the air core of the transformer, the magnetic field is uniform along the radial, but a little higher near the rim. Besides, the magnetic field decreases relatively faster along the axial when it approaches the edge of the coils. Finally, an air-core pulse transformer with a coupling coefficient of 0.8 is constructed, and the magnetic field space-time distributions in the air core and the area between primary and secondary coils are measured. The experimental results correspond with the theoretical simulation results. The