In this paper, to calculate overvoltage in air-cored transformer windings under very fast transient overvoltage, firstly, the voltage transfer functions are calculated which are measured by network analyser or calculated from the distributed parameters of transformer windings. Secondly, the voltage transfer functions are fitted with rational functions by vector fitting codes and then the rational transfer functions are order-reduced by the Pade approximation. Lastly, the resultant voltage transfer functions are synthesized by the elements in Electromagnetic Transients program. The calculated voltages from the circuit by Electromagnetic Transients program are in good agreement with the measured voltages. This shows that the method is simple and robust.