Abstract: With the rapid development of the offshore wind power, the traditional columnar air-core reactors increasingly present their prominent disadvantages. Because of the open magnetic fields of the reactors, they are not conducive to the compact design of the offshore converter station. The toroidal air-core reactors can effectively reduce the actual space occupation by restricting most of the magnetic field inside the coils. In this paper, a toroidal air-core reactor prototype is developed, its wideband equivalent circuit model is established, and the circuit parameters such as the inductance and capacitance are extracted. The transient voltage distribution of the prototype under the action of 1.2/50 μs low lightning voltage is measured and calculated and the validity of the model is verified. The results of analysis indicate that both the node-ground voltages and the voltages between coils oscillate. Take the peak value of the head voltage as the reference value. When the end is floating, the node-ground voltages increase gradually from the head to the end and the max voltage is 1.765 pu When the end is grounded, the overall level of voltages between coils is higher than that when the end is floating, and both of them show a tendency of being low in the middle and high at the both ends. The max voltage between coils occurs at both ends of the first coil when the end is grounded, which is 0.109 pu The max interturn voltage of the first coil occurs between turns 5 and 6, which is 7.88×10⁻³ pu Finally, the influence of the structural parameters on the maximum transient voltage is discussed, and the measures to improve the insulation performance are proposed.