The technical scheme of the invention discloses a fixed switch cabinet partial discharge online detection device and method. The online detection device comprises three subsystems of a signal transmitting system, a data acquisition and processing system and a diagnosis and alarm system. A software system adaptive to the online detection device is developed on the basis of hardware, and a series of functions of data wireless transmission, effective signal denoising and instant short message alarm can be realized through the integrated design so that detection of partial discharge of all switch cabinets can be completed, and rapid short message alarm for the certain switch cabinet of which gas concentration or gas relative generation rate exceeds the threshold can also be performed.