The invention discloses a fault detecting device and method for a switch cabinet. The fault detecting device comprises a first detector, a second detector and a processor. The first detector and the second detector are both connected to the processor, and the first detector is used to detect concentrations of CO and NO2 in the switch cabinet to be detected, the second detector is used to detect photosensibility and sound pressure level in the switch cabinet to be detected, and the processor is used to receive the concentrations of CO and NO2 and judge whether or not a fault exists in the switch cabinet by comparing a calculated quotient of the CO concentration to the NO2 concentration to be determined with the preset quotient and determine the type of the fault in the switch cabinet, and then judge whether or not the fault exists in the switch cabinet according to a detection result from the second detector, ensuring accuracy of the judgement result. The fault detecting device of the invention can realize real-time on-line monitoring, not only detect the fault in the switch cabinet but also judge the type of the fault.