The invention discloses a large pump unit on-line monitoring and diagnosis system. The large pump unit on-line monitoring and diagnosis system comprises an air gap sensor, an electric eddy sensor, an acceleration sensor, a temperature sensor, a signal conditioner, a signal processor, a WEB server, a data server, a user client, a fire wall, a router and a remote expert sub-system. Information collected by the four sensors is conditioned through the signal conditioner and then sent to the signal processor, data processed through the signal processor are transmitted to the WEB server, the data server and the user client, the data of the user client are checked by a user, and the data of the WEB server are sent to the remote expert sub-system through the fire wall and the router; the remote expert sub-system provides corresponding maintenance measures according to the operation state of a unit. The large pump unit on-line monitoring and diagnosis system can monitor the operation conditions of the unit in real time, monitor the changes of the air gap and the magnetic flux of a motor, predict faults of the unit, shorten the stop time and reduce the overhaul cost.