Abstract: The water penetration of cable terminals in the switchgear of underground substations is one of the frequent hidden dangers in equipment operation, which has a significant impact on the insulation of the equipment inside the switchgear and must be addressed. This article takes an underground substation in Beijing as an example to analyze the causes of water penetration in the switchgear, including water in the cable trench, cable sheath damage, cable terminal sealing in the switchgear and other factors. This paper puts forward four preventive measures to provide reference for solving similar problems.