The invention discloses an online monitoring and fault diagnosis system of large wind turbine units. A sensor and data collection equipment are installed on a wind turbine unit to be detected, so as to collect equipment operation state characteristic signals, such as video signals, noise signals, flame signals, electric signals, vibration signals and pressure signals, of the wind turbine unit, and transmits the state characteristic signals to a central control room of a wind power plant through a network in real time; and operation and maintenance personnel can know the operation state of the wind turbine unit at any time in the central control room of the wind power plant, and can directly analyze signal data of the abnormally operated wind turbine unit, so that operation conditions of gear boxes, main shafts, generators, bearings, hydraulic systems, yaw systems, bearing lubricating systems, brake systems, blades and towers of all units in the wind power plant are monitored in real time and failure warning information is output. Data can also be transmitted to a remote fault diagnosis center through a network, a project team performs professional analysis to faults and issues a diagnosis report, and a field maintenance team performs corresponding repair/maintenance operation.