The open-core voltage transformer has the advantages of flat U-I curve and good anti-ferroresonance performance, but there are still few studies on the electromagnetic characteristics of it. In order to promote the knowledge and understanding of the electromagnetic characteristics of the open-core voltage transformer, the difference between the open-core voltage transformer and the closed-core voltage transformer on the U-I curve and the reason why the open-core voltage transformer has excellent anti-ferroresonance performance are analyzed from the theoretical point of view. Then, the core parameters including the shape and material of the core and the number of winding turns are selected as the influencing factors, and their influence on the error of the open-core voltage transformer is studied by simulation analysis. Finally, based on the simulation results, some suggestions are put forward for the design of the open magnetic circuit voltage transformer.