Abstract: The leakage rate is an inherent property of hermetically sealed electromagnetic relays. When used in the space vacuum environment, the relay’s inner gas will progressively release and its inner pressure will be reduced gradually. As a result, the relay’s contact points may be switched into a low pressure environment and the contact failure may occur. In order to verify such mechanism, some homemade relay samples are selected and the contact switching experiments are conducted. The arc failure laws of the relay’s contact point switching in a low inner pressure are studied.