As one of the main application topologies for medium and small auxiliary power supplies, the input of the flyback converter needs to meet the requirements of wide range and high voltage. However, due to the parasitic parameters of the transformer, the flyback converter has a certain current spike. At high input voltages, the current spikes cause large converter losses, affecting the conversion efficiency, power density, and reliability of the entire device. Therefore, this paper analyzes the cause of the current spike of the flyback converter in detail, and proposes the improvement measures to suppress the turn-on current spike from the two aspects of transformer winding layout and winding interlayer connection, and develops the input voltage range of 300~3000V85W. The experimental results show that the converter efficiency can be improved by 7% by improving the layout of the transformer windings; the converter efficiency can be improved by 2% by changing the connection mode between the transformer windings.