Strong electromagnetic radiation is produced when the three-electrode gas spark switch works. In this paper, firstly, theoretical analysis of the on-current and electromagnetic radiation of the switch is carried out, then the switch is simulated by CST software and the breakdown field strength of which is calculated. Secondly, the on-current and far-field radiation field strength of the three-electrode gas spark switch are measured experimentally, and the results are summarized and analyzed. Finally, the strong electromagnetic radiation of the switch is suppressed with electromagnetic shielding method effectively. The research results can provide reference for electromagnetic radiation and protection of pulse power devices.