The invention discloses an online monitoring system and a method for partial discharge of an intelligent switch cabinet based on ultra-high frequency detection. The online monitoring system comprises a sensor unit, a signal conditioning unit, a MCU (Micro Control Unit) unit, a communication unit, a system power supply and an upper machine monitoring unit. The online monitoring method comprises the following steps that: the sensor unit finishes collection of partial discharge signals in the switch cabinet; the signal conditioning unit realizes amplification, filtering, frequency reduction and bias of the signals, thus obtaining signals which can be converted by an ADC (Analog to Digital Converter) module in a DSP (Digital Signal Processor); the MCU unit realizes analysis and procession of the signals; and data are uploaded to an upper machine in a monitoring room through an Ethernet. According to the online monitoring system and the method for the partial discharge of the intelligent switch cabinet based on the ultra-high frequency detection disclosed by the invention, collection and procession of high-frequency signals conducted by the DSP are realized by adopting a frequency spectrum shifting technology; a time-frequency analysis technology and a mode identification theory are applied to the monitoring system, therefore, the monitoring system is functional in fault diagnosis; and in addition, an IEC61850 standard is adopted by the system, consequently, management of various IEDs (Intelligent Electronic Devices) and interconnecting as well as networking of equipment in a transformer substation are convenient.