This paper presents an electromagnetic simulation method for spark-gap switched pulse generators (SGSPGs). Electromagnetic simulation programs can be useful for designing nanosecond or subnanosecond SGSPGs; however, these programs do not support simulation models of spark-gap switches. This paper describes a simulation method that determines electric field at an observation point in the SGSPG by replacing the spark-gap switch with a step voltage source. In this simulation, the discharge properties of the spark-gap switches, including the breakdown voltage and rise time, are determined using the results of the studies on electrical breakdown. A 1-ns single pulse-forming line was fabricated to validate the proposed method. The simulated waveform was in good agreement with the measured waveform.