The invention relates to the field of electromagnetic transient simulation, in particular to a switching state judgment method and device, a parasitic switching judgment method and device, and an electromagnetic transient simulation method and device for applying the switching judgment method to a half-bridge voltage source converter. The technical problem that in the prior art, a switch judgmentalgorithm can cause huge simulation time consumption is solved. The method is based on a half-bridge voltage source converter widely applied to an electric power system, is different from a switch judgment method in traditional electromagnetic transient simulation with independent switch elements as basic units, and takes independent half-bridge sub-circuits forming the half-bridge voltage sourceconverter as basic units for switch judgment. The switching judgment process comprises the following steps: carrying out Norton equivalence on each half-bridge sub-circuit; and judging the switching state of each switching tube/diode in the half-bridge sub-circuit according to the stable state of each independent switching tube/diode switch in the previous time step and the switching tube gate signal, the terminal voltage and the branch current in the current time step.